I want to know if I can use a flush valve as part of a water change system.
I could pair this with the floating thing that automatically fills up the tank, and this can also act as an overflow, so I can change the water in a press of a button.
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To answer the question, "Can I?" the answer is probably, "Yes." It would physically work to remove water and automatically replace it.
But, there is another question hidden here, "Should I?" And the answer to that is most definitely, "No." Here's why:
Flush valves are designed to empty the cistern completely. You would need a way to make sure that you don't leave your fish dry while your tank fills up. Your heater would also not survive.
You would need a way to ensure that you don't flush your fish with the water.
One of the advantages of siphoning water out of the tank manually, is that you're able to target the areas from which water are removed, and in so doing, target the waste that is in the tank. If you were to use a flush valve, the waste would still be lying on the bottom of the tank. That would mean your water change would not be effectively removing the waste.
Using the float valve to fill the tank would kill your fish. Tap water needs to be treated to remove chlorine and other chemicals before going into your tank. The tap water would also be cold, and the temperature drop would, without a doubt, shock the fish.
Making holes in your tank is risky. It is easy to crack the glass and ruin the tank. Even if you get the holes made successfully, you're adding weak points to that tank where leaks could develop.
If you could overcome all of these challenges, then maybe it will work, but the effort required to achieve this would probably outweigh the benefit that it would provide.