i'm only 7 to 8 months into this hobby, but less than a week old with the realization that there's a whole science to it, thanks to this site.
i have 2 70 to 90 gallon freshwater tanks. i'm not really sure because i had them custom-built to fit a corner in the house. they measure 72in wide, 18in tall and front-to-back is 14in. so how many gallons are those?
the top tank has 4 snakeheads (~10 to 12in), 6 red pacus that are actually silver in color (~5in), 3 black oscars (~8in) and 1 black janitor fish (~couldnt care less). this tank started with more kinds and the above fishes are all thats left. call it evolution or survival of the fittest, let's just say i learned fish compatibility the hard way. i now have a smaller tank- 48x14x14, that houses goldfish that are actually meant to be live food (had i known!!!) and some fishes that survived the nasty trio above - 2 knife fishes (1 missing an eye), 2 dragon fins, and 1 black snake with white markings.
the lower tank has an assortment of guppies, dainios, barbs, dwarf red & blue gouramis, redtail 'sharks', crayfish (??? thats what the store told me), albino janitors, black and orange mollies, etc.
like i said, i had no idea on proper aquarium care, much less what fishes to combine. all i have are overhead filter boxes that have those foams, carbon bags, and another bag with what looks like white lifesavers.
i change 80% of the water on the top tank every 2 to 3 days because those 3 fishes are freakin dirty and they make the water gray really quick. i removed the pebbles, stones, etc and now the bottom of the tank is just glass. makes it easier for me to vaccum their waste. no i don't check for anything in the water nor do i add anything. its just plain philippine tap.
the lower tank i change 80% of the water every 2 weeks. same thing, no checking and nothig added. removed the underwater white plastic things and just have pebbles and stones. i leave the fishes in the tank when i clean. i plonk the stones in there right from the store. needless to say a whole lot of fishes in the lower tank have died and have eventually found themselves in the bellies of the snakeheads.
i realize i've been a bad bad fish owner after discovering this site. am now on the road to repentance.