My fish are all dead :( My snail was the last to die last week...
I'm not sure what went wrong in my tank though. Everything (over time) was covered in a black, hard-to scrub substance...even the leaves of my live plants (moneywart grew great in my tank/anubias nanas so-so). There is brown slime in my filter and on my tank glass every week that I scrubbed off. I cleaned and replaced water weekly, replaced my filter cartridge/those carbon things inside regularly, etc.
I started up my 20 gallon planted tank almost a year ago, but my fish (platys & mollies) never stay alive over 3-4 months. I fed them once a day and added more root tabs for the plants, as needed.
I am on well water, which is high pH but runs through a water filtration system and softener before coming out of my tap. When I run water tests, I'm good on nitrates/nitrites/etc. I add aquarium salt and a little conditioner with each water change to play it safe. I have a bubbler in the tank to help with oxygen already. The hood has LED lights (~ 6 watts, which are on for 12 hrs./day), and I use a Top Fin Power Filter EX20. It stays about 78-80F. I use small gravel substrate.
I want to fix my tank before I shop for new plants and fishies, but I am not sure where to begin.