The Fish Cave - Nearly Automatic Aquarium Water Changer - DIY

TheFishCave Aquarium WaterChanger (Previously called Mathew's Waterchanger)

These waterchangers were an idea of mine that I had a little over 4 years ago. They took a further year to fully test, develop and then another 2 years to put onto all of my tanks. Oh yes and in those last two years I also built a fishroom, but that is another story.

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Matthew’s WaterChanger or TheFishCave Aquarium WaterChanger
The way to drastically reduce the time you spend on routine maintenance


The concept behind TheFishCave WaterChanger is very simple. It works on the principle of a normal everyday siphon. We siphon water out of our fish tanks on a regular basis. TheFishCave Waterchanger allows a permanent syphon to exist 24 hours a day, ready to drain the water whenever we decide that the time is right to perform an easy water change.

To understand how the WaterChanger works, picture yourself syphoning a tank into a bucket.
If you raise the bucket up to an equal water level then the flow into the bucket will stop.
If you lower the bucket then the flow will start again.

TheFishCave WaterChanger works on the same principle. Except the bucket is now made out of 25mm Pressure pipe. If you raise the 25mm pressure pipe (bucket) up above the tank’s water level then flow out of the tank stops, when you drop it below the tank’s water level then the water drains out of the tank. Simple...

Water Changers in Operation - Red Cable ties for demo purposes only

Full Tank with Water Changer Normal

Water Changer Empty Drain

Water Changer Overfilled Tank Filled

Normal water level inside your tank
(They normally live behind the tank!)
Water Changer has been lowered and the water has drained out The tank has been overfilled, but the extra water will safely overflow and drain away.

So by moving the WaterChanger up or down we either start the WaterChanger transferring water out of the tank or we can stop any further water from flowing out of the tank, by lifting it up.

So what do the components look like?

U Bend Completed U Bend
Parts for a Water Changer

This is the movable WaterChanger component.

It is made from 25mm pressure pipe, an end cap and a Cat 21 Faucet Tee. The other component is the screw in elbow that is a Barbed x Threaded Elbow Male 19mm X 20mm BSP. These fit very loosely into the tee and must be screwed in with a very generous amount of teflon tape.


How big a Water Changer do you need?

The measurements. Here is where the mind starts to muddle when you consider the calculations involved. However fear not it is easy and absolutely possible. The factors you need to consider are:
How much water do you want to remove from the tank, as a maximum?
How much water do you regularly want to remove when doing waterchanges?
Can the water changer drop below the level of the base of the tank?

With this information you can work out what lengths the pressure pipe is to be cut to.

Generally I made the bottom section of pipe at least 50mm longer than the top section. Coincidently this meant that my maximum water removal was around 40-45%. PS. No need to panic, the % of water change can very easily be reduced. So if you want to do only 30% on a regular basis this is no problem at all.

One word of warning, please keep in mind that you will need to cut the bottom part longer than the top piece of pipe, if not you will find that you can easily break the seal and will need to reprime the waterchanger. Thus the bottom section must be longer than the top.

After you have your measurements cut the components and loosely fit them together.

You can build them whichever way you wish, but you must always maintain the seal (or syphon) otherwise you will need to reprime the U-Bends.

The U-Bend

One key component to all of this is the overflow pipe or “U Bend” that is made out of 20mm electrical conduit. This pipe allows the water to flow from inside the tank and into the 25 mm WaterChanger section.

How to bend the 20mm Electrical Conduit and make a U-Bend

Bending Spring and Pipe For this you should use a 20mm electrical conduit bending spring. These are available from your electrical Wholesaler and cost anywhere from $3 - $5 depending on how well you get along with the staff there and whether you can access trade pricing.

Other tools required for the bending spring. Heat gun, a $20 unit will do the job nicely, you could try a hair dryer but I have not tried it myself.

Bending Pipe

Insert spring into a piece of pipe you believe will be long enough. Eg. I regularly cut lengths that were 1m long for 18 inch high tanks.

The next thing to determine is how wide you need the bend to be. Put you waterchanger beside the tank in the position that it will normally be placed and then measure the inside measurement. Then I cut pieces of scrap timber to the desired bending width. Then simply bend the pipe around the timber to get the required size bend. The timber is held in a vice.


Heat the pipe slowly. If you direct too much heat at one spot you will burn the pipe and it will fail when you bend it.

After you are happy with the size of the bend remove the spring and quench. (Place into a bucket of water). I have no idea why this works but it seems to hold its shape a lot better as opposed to simply letting it cool down slowly.

Fitting the U-Bend

Fit the 25mm section into its normal resting position, then try slotting the U-bend into it and the tank. You are aiming to get the U-bend as deep as possible into the WaterChanger, whilst minimising the height of the u-bend itself. Because the higher the U-Bend, the slower the waterflow out of the tank.

After you have gotten this measurement correct, work on the end that will reside inside the tank. This end needs to rest approximately 50mm from the base of the tank/substrate. The reason being if something bad happens and your tank drains, due to some problem then the fish will still have approximately 50mm of water to survive in until you come to the rescue.

After you have gotten a measurement of around 45m from the bottom (No this is not a typo, keep reading) you need to make the  tank end of the pipe into a strainer. Unless you have seriously big fish of course.

Building a strainer

I looked at buying a commercially available strainer however to buy 65 at $6 each seemed a bit muck, so I came up with the following method.


Clamped pipe

Using a heat gun heat up the end of the pipe and crimp the end tightly in a vice. After say 15 seconds quench the end in a bucket of water. If not you will find the end tends to separate.

Cutting Strainer

To cut the slots into the crimped end use a hacksaw that has two blades in it. Place each blade in a different direction. To hold it together and give a good sized slot, sticky tape the blades together, don’t worry about cutting through the tape, as it lasts a surprisingly long time ad you will know as soon as the gap is too big.

I generally aimed to make 5 slots into the now crimped end but often only manage to get 4 into it. The difference is not a problem, so fear not.

Now to the other end of the U Bend pipe you need to make a cut at something like 45 degrees, this will allow the water to enter the pipe and drain away. If you forget this step you will find that the water drains very slowly.

Next simply insert the pipe into the water changer onto the tank and you are good to go!

Ooops, hang on a minute, how do you get the water up into the U-Bend?

Pump and fitting it

This problem took me a little while to solve, but in the end I used a pump. The pump forces water up and through the WaterChanger and expels any air present. By the way you don’t have to do this ever again, well not until you allow air to enter the U-bend. Have a close look I just carefully heated and bent a piece of pipe so that it fits over the intake strainer of a U Bend.


Once you have the water in the WaterChanger you need to get it into a drain. This is done by attaching some 19mm irrigation pipe to the water changer and the other end into a drain or something that will take it to a drain.

So where does the water go after it has been removed from the tank via the WaterChanger? Into a drain of course. If you are not able to build your fishroom from scratch, you can use some 90mm pipe stormwater pipe on the floor of your room and run this to the nearest drain. To make an entry spot for the water to drain into use a 90mm tee. Of course if you use this method you will need to place this behind your tanks or the tank racks. This is the preferred method as it allows the precise placement of the drains that will line up exactly with the waterchangers that will be above them. You may be surprised at how badly your measurements are so a built into the floor drainage system may not be the best option. However I will say that building in a few floor drains will make your life a lot easier for the 90mm pipe to drain into.

The reason that 90mm pipe was chosen is because it is the cheapest option in large pipe, the tees are also very inexpensive. Furthermore it allows a large amount of water to drain very quickly out of the tanks, via the waterchangers into the floor drain.
If you are unable or unwilling to use 90mm pipe, then you can use a smaller grade of pipe or use 19mm irrigation pipe. The pipes all of course lead to the drain. The down side to using 19mm irrigation pipe is that it slows the draining of the water our of the tanks. On the positive side it is also one of the cheapest options. So the choice is up to you.

Refilling System
The next component that is absolutely optional to the WaterChanger, is the refilling system. As the name suggests it allows you to refill your fish tank using a system that is permanent. It can also be made a lot easier than using buckets if you do not want a permanent refilling system attached to your tank.

If you are building a permanent refilling system I recommend that you create a loop that goes from wherever you water source is (Barrels or whatever your water source is), to the area where your tank(s) are and then back to the water source. This design allows the water to be pumped either into tanks or back into the water source. The reason for doing it this way is so that when you can turn off the valve that is attached to the tank the remaining pressure will not cause any leaks or cause taps or connections to get blown apart.

It can be a time consuming exercise to install a permanent refilling system, especially when it is built out of pressure pipe, however once it is done you can look forward to many many years of trouble free and quick refilling of your tanks. Nothing beats being able to turn a tap on and have water pour out into a tank.

If you are happy to roll out a length of pipe when you perform a water change then there is nothing wrong with doing a waterchange this way, simply attach a powerhead or pump onto the end of a length of pipe that will reach from your water source into your tank(s). The best way to do this is to make a hook at the end that attaches to the end of the hose. The hook is made out of the pipe and two irrigation elbow fittings. The hook is used to stop the pipe from falling out of the tank. For a detailed description have a look here.

Attaching a WaterChanger to a tank
To attach it to a single tank, have a look at the picture below.

Water Changer

I have used red cable ties so that it easily shows how it all holds together. Of course for your own system you should use white cable ties as these look much better.

Top Water Changer Clip

The top ‘clip’ is made from the base of a broken milk crate. It can of course be made from whatever you believe will do the job and not break. Plastic is the obvious choice as it is unlikely to break, will not rust or rot and is unlikely to cause your tank to crack.

Bottom Clip

The lower component is made from half of a plastic clip that is used to hold 13mm irrigation pipe onto timber. I simply cut the rest of it away.

Attaching a WaterChanger to a rack of tanks. This was a bit of a challenge as many new factors come into play.
I was considering all sorts of things like pulleys,rope and even some kind of an automatic/electrical system.

Water Changers on a rack

This is what I did, feel free to copy and or modify it. Maybe you can think of a better method? PS. I did.... have a look below.
PS. This photo is of one of my prototypes.

Timber with Water Changers

I decided to attach the WaterChangers to a piece of timber. This piece of timber when moved would allow the water to drain out when I wanted to change water. I designed the WaterChangers so that the top of the 25mm pressure pipe would be inline with the very top most point of the glass in each tank. By the way this means that whatever the irrigation fitting is thus placed means that this is the maximum amount of water than can be removed from the tanks. This made it easy for me to work out where to attach the WaterChangers to the timber. As I simply placed the timber up against the tanks and drew the line at the top most point of the tanks.
PS. This length of Waterchangers were not designed for the tanks shown in the picture.


To work out how high to move the timber was also pretty easy as all you need do is move the WaterChanger up until you reach the topmost point where you want the water to be in the tanks. By the way this I worked out would be the spot where the embedded tee is in the WaterChanger.



. In order to get the WaterChanger to move into certain levels proved to be a bit of a problem. I was thinking of all sorts of complex levers and graded slots to slot a piece of timber that jutted out from the upright piece of timber. In the end the solution proved a lot simpler. I just attached a bolt onto the handle that jutted out form the upright piece of timber and to this I attached timber that was cut to the correct lengths that I wanted. These piece of timber are movable and allow the timber to drop down.


Variable Height levers

You are also not limited to a set amount of water to remover from the tanks. Simply attach another piece of timber that is cut to a different length and this will allow things like a 10% water change.

It takes a long time to get this part built and correct but I assure you it is the best thing I have ever done as one handle can move anywhere from one, two, three, six or in some cases 8 WaterChanges, all at the same time.

I hear you saying hang on, what about if I only want to do a water change on a single tank.
Then you have three options.

  1. Do it the old fashioned way, with a siphon
  2. Break the siphon in the U-bends attached to the other tanks.
  3. Use the waterchanger as an overflow box, simply fill up the tank and let the waterchanger drop the water level back down to the normal level.

Personally I use a combination of all. You will however have to refill the U-Bends if you allow air to enter them, no big drama of course.

If you decide to build a WaterChanger then here is your shopping list. The total cost for one WaterChanger is around $8. This includes the tap that is used to refill water back into the tank after the tank water has been removed.

Barbed x Threaded Elbow Male 19mm X 20mm BSP         
PVC Cat 6 Cap 25mm                                                           
PVC Cat 21 Faucet Tee 25mm X 25mm X 20mm   
Electrical Conduit 20mm

Refilling system – Optional but I highly recommend you do this!
Pump or Powerhead, Bigger is better
19mm Antelco "Green Back" Valve or 13mm Antelco "Green Back" Valve
Irrigation pipe to suit your vales & Elbows & Tees

20mm Bending Spring, Teflon Tape, PVC Glue, Hacksaw,
Clear PVC Primer(Acetone will do), File, Tape Measure
19mm irrigation pipe about $8 for a 30m roll
13mm irrigation pipe about $4 for a 30m roll

I wish you all the luck in the world on your project and I am happy for you to copy my system.

Complete system

Here is a picture of a system as it is being installed. You can see the Waterchangers are already in position, just waiting for the U Bend tubes. Also the 90 mm drainage pipe.

I wish you all the luck in the world on your project and I am happy for you to copy my system.


All I ask is that if you do build the system please send some pictures and include all of the modifications that you have made on my design. I am positive that it can be built better!

All of these pics and modifications will be posted up including links to any locations where they may be found on the web.

MatthewTech {@}

Good Luck
Matthew Begaud

Disclaimer: These articles are intended to inform and educate. Always use safety equipment and please be extremely careful. By building any of these DIY projects you agree not to hold the author or the owner of this Website responsible for any injury or bodily harm you may cause to yourself or others.

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