Automatic Weekly Water Change

A few months ago I bought a 125 gallon aquarium from a colleague. After realizing that I was never going to keep up with regular water changes on this beast I decided to search for an automatic water change solution.

I examined the designs of many DIY auto water changers but quickly realized that nobody had in mind what I needed to do. Because I keep plants I didn't want a trickle system or one that continuously replenishes the water. I wanted the system to perform a single 25% - 50% water change at the end of each week. The solution I came up with is simple and didn't break the bank.. although it wasn't super cheap. The cost is comparable to ready made auto-top-off systems.

I should mention that I had several advantages. First, the aquarium is installed in the basement merely 20 feet from the laundry room which has a utility sink. Quite convenient. The second is our basement ceiling is unfinished so I can string anything across the ceiling I want and it's not going to look any worse than it already does. You may or may not have similar advantages in your home but even so with a little forethought I believe anyone can make this work with their aquarium.

You will need the following:

- One Dual Switch Sump Mount with Latching Upgrade from www.autotopoff.com. Note: it MUST have the "latching upgrade" because that is the only thing which makes this work. The latching circuit makes the two switches act as one large On/Off switch to drain all of the water between the two switches.
(photo) http://autotopoff.com/products/DS1/pages/DS01_JPG.htm
$60.00

- One Solenoid Valve with compression fittings for 1/4 in. OD nylon tubing
I purchased mine from www.autotopoff.com as well but these are available elsewhere if you know what you're looking for.
$31.00

- One Digital Timer with daily settings. I bought the cheapest one I could find.
$18.00

- One length of 1/4 inch outer diameter tubing. I needed 25 feet.
$4.75 @ $0.19/ft.

- One length of 1/2 inch inner diameter tubing, or the proper diameter to fit your pump or powerhead. I'm using an Aquaclear 301 just because I had one.
$14 @ $0.56/ft.

- Fittings to connect to your water supply. In my case I had to reduce from 1 in. -> 1/4 in. diameter.
$15.00

- A ball valve (or similar) to adjust flow from your water supply to the solenoid. I found that this was necessary because although the solenoids can handle a lot of pressure the water speed going into the aquarium was too great.
$8.00

Total cost:
$150.75

Here is a diagram of how it works...


The water change begins with the timer. The timer turns on the pump at the given hour on the given day. The timer is set to stop the pump after slightly overshooting the bottom float. You'll have to test this. In my case its 27 minutes. Once the water level falls below the bottom float it triggers the solenoid valve to open and refills the aquarium. When the water level rises above the top float switch the solenoid valve turns off and the water change is complete. SIMPLE!

This auto water change solution is ideal for freshwater planted aquariums that are more stable with weekly water changes rather than continuous water replenishment, but will work well with any tank.


Here is the drain pump, fill return and float switches mounted in the aquarium. Just ignore the blue thing. The tank is just being set up and I thew in an air stone for the fishes.


Everything connects to the ceiling. Actually this photo does not show the power cord which connects from the control box all the way back to the sink where I have installed the solenoid just in case something pops a cork and decides to flood the basement.


The drain pump connects to a PVC pipe on the sink just to guide the water down. The solenoid is currently hanging on the inside of the front of the sink. Later I'll mount everything more neatly.

And that's it!

Hopefully this inspires you to do your own auto water change or improve on this one. If you have any questions let me know.

This system really isn't good for auto dosing. That's all still done manually dosing directly to the aquarium as ussual. It's possible you could use a solenoid valve in combination with a metering valve (such as those used for CO2 dispensing) to precisely dose dechlor and nutrients but you'd need a really good timer.

If you were thinking about "water aging" which some people like to do for particularly sensitive fish then this system can easily be adapted with that in mind. Here's a diagram..



In this configuration the timer still activates the drain pump in the aquarium but instead of the aquarium float switch activating the solenoid it would activate the pump in the holding tank. The switch in the holding tank would activate the solenoid refilling the holding tank. In this scenario the water would sit for a week or for however long you desire between water changes.

I figure it would cost an additional $75 or so for the additional float switch and pump.

I have long used untreated tap water trouble free. Frankly there are other things that concern me more than the local treatment facility suddenly and in inexplicably tripling the amount of chlorine/chloramine used to kill bacteria.

Photo of the 40 gal in my sig.




The plants and fish in this tank are swimming in untreated tap water and have been for several years. However I recognize that not all tap water is the same. And so as a solution you can extend on this design to add a water aging tank. Personally though I have not had the need.

I haven't tested the temp swing yet. I know the fish don't mind and I don't anticipate any problems with the plants. It takes a good 50 minutes for the system to refill the tank so it's not the same as dumping in gallons of cold water all at once.