Good or bad idea? I have a couple of healthy freshwater tanks that have been running for 10+ years. Lots of beneficial bacteria. Can I make teas with it? Use it straight or add a few things to it?
Just do it. I did it with trying the guppieponics thing years ago in a soilless mix and it did ok. In soil I believe it would be an excellent factor in keeping the myco population healthy. Edit: If in soil I wouldn't sweat watering it down or watering or anything. When I was raising misquito guppies I kept the ph at 6.8 and the ppms measured at 600ppm max. Not that any of that matters, it's just a healthy fish tank has happy water.
Poppy is spot on. I always use my waste aquarium water as a water source for my act. Why waste it? That's why aquaponics works. I also use it to water clones. Recycle that water. I run my tanks slightly acidic with peat and drift wood.
I throw the used carbon filters in my worm bin and I use the water in my teas when I clean my tanks. I only did it twice so far and I see no ill effects but having only done it 2 times I havent really observed any positive effects. Not that there isn't, Im sure all that fish poop water brought some diversity to my soil microherds and the added carbon in my vermicompost.