Please please please please PLEASE tell me that your 5.5 gallon tanks are actually 55 gallons and there is a typo in your post and just really large fish. Please. Otherwise, your tanks are all incredibly overstocked. Most if not all of those fish are unsuited for anything less than a 10 at the very least. Zebras really need a 15 gallon or more due to how active they are. Harlequins Neons, and Black Neons(yes they are different species) need a 10 gallon. Bristlenoses need a 29 gallon. Along with that, a lot of those fish need at least 6 or more of their own species (the danios, harlequins and neons) due to their shoaling/schooling nature. Just because your ammonia levels never spike, doesn't say anything about the actual ammonia, nitrite and nitrate levels (ppm) in your tank, nor the actual stress the fish are under due to lack of space within the tank. How much water are you changing every 4 days?
The slime's probably a fungus, it'll go away eventually.
So many fish/plants/inverts to keep, not enough aquaria.