I have two twenty gallon tanks. My oldest tank, tank 1 has live plant friendly gravel & at one time i was running a cheap Co2 thingy for my plants. The other tank,tank 2, approx 4-5 months younger has a sandy gravel and has had no Co2 thing set up ever. All fish in both tanks are doing very well. These include, cory's, plecos, tetras, rainbow fish, a platy and a guarami. Tank one is over grown in plants (even though the co2 hasn't been running in months). I mean my java fern and some other plant that was 4" tall are both HUGE...BUT...here's the thing. I have an overgroweth of this black soft fuzzy algae. I tried to clean it off the wood and stones I have in there but it survived and is everywhere. As I said all the fish are fine, have grown etc. BUT my apple snails are not growing or thriving well. In fact some have gotten thin, peeling shells and have died. On the other hand, tank 2 has the same brothers and sister apple snails as from tank 1 (we actually had their parents) and they are HUGE and very very healthy...and the plants are the same scrawny things i put in there 3 months ago...and I am also overrun with those little nuisance snails in that tank. Fish are great in tank 2 also. Can anyone explain this to me? I am wanting to downsize to just one tank. Which should I keep? I even contemplated combining them (water) but tank 1 has the black algea i don't like and tank 2 has the little snails i don't like and also tank 1 has some problem that obviously hurts my apple snails and i don't want to hurt my healthy ones on tank 2. I have 2 tanks and am not happy. Any advice would be appreciated.