That is clever keeping some critters in the quarantine tank. They may be vulnerable to what comes in with new fish. And if an apple snail dies, you really want to quickly lift or siphon the remains in that shell out quickly. That is not a pretty task, but it is really essential to get all that dead organic stuff out of the aquarium system fast.

Your first fish, in tank # 1, obviously weren't quarantined because that was your first tank. New tanks are "de facto" quarantine tanks.

Fin-atics like myself, who will set up a small (10-gal/ 39 liter) tank for new acquisitions, are also setting up de facto quarantine tanks with the new fish. Of course if the fish "stick," they are then in a single species tank where successful spawning and survival some youngsters is much more likely.

Your pH is a little low for guppies, though I am sure they encounter that in some places, some rainy seasons. The San Francisco area, in part, has water with a modest enough mineral content that aquarists regularly spawn killies, tetras, dwarf cichlids and other rain forest fish which other aquarists really need to work with elsewhere in California. I would kill for that water in Chicagoland.

Rather than pH adjusting chemicals, maybe leave some coral gravel in the filter or on the tank bottom. I know there is a problem with lighter gravel causing colors to wash out though.

People raising rainbowfish fry have discovered the need for a pH over 7. The coral gravel is one of the easier solutions and the gravel seems to "coat" a little slower than other alkaline materials. A modest, measured quantity of cichlid salts (or the homemade cichlid salts) or the more expensive (Sea Chem) Equilibrium in the new water might also be useful and safer than the pH adjusters.

The combination of plastic and real plants is something more aquarists might consider. That is a creative adaptation to the realities of many aquarium lights and hoods (which are more "reasonably" priced, but correspondingly under-powered). Often there is not enough light for every plant one might like to keep, but some live plants can be effectively kept and even reproduced. Even the big professional aquariums may use that combination in some of their set-ups, from time to time.

"1 fat bottom feeder of unknown id!" is a little disconcerting. Probably safe if it were one of the suckermouthed catfishes. If however, it is a catfish, but not a Loricariid, I would be careful. If it has long barbels/feelers and/or large eyes it is a night feeder (among other things). A number of cute little catfish, with relatively big mouths, have been responsible for the mysterious disappearances of other aquarium fish. You didn't say what kind of tetra was in there with him. But some tetras and livebearers certainly can be vulnerable to growing night-feeding catfishes. (And let me tell you about American eels sometime!)

In your spare time (this time of the year?) you might like to browse http://www.planetcatfish.com/core/index.php
What we don't know CAN hurt our smaller fish. :)