Up until about 2 weeks ago I had a perfectly happy healthy 20 gal tank. I had 3 guppies (one very pregnant), 3 Glo tetras, 3 black skirt tetras, 1 powder blue gourami, 1 albino pleco, and some inherited snails from the plants. I am running a Eheim canister filter and a bubble/LED wand that's on a timer and the tank is equipped with a LED day/night light.

My pregnant guppie gave birth and only one survived. A couple of days later one of my male guppies (the older) died for no apparent reason. I was having trouble reading the temp on my therm strip so I replaced it with an in tank type. I noticed that the temp read outside of the "safe zone" marked on the thermometer so I gradually (one or two degrees a day) reduced the tank temp so that it now reads at the top end of the "safe zone" (approx 78/80 degrees). A couple of days ago I noticed a white something on my now pregnant again female guppie so I went to Petco (there aren't any local fish stores that carry fresshwater fish in my town so my choices are limited to Petco or Petsmart) and picked up Primafix because it said it would fix what I though my fish had. Well I've been following the directions and the white seemed to be disappearing but today we woke up to find the female guppie dead at the bottom of the tank. Last night we also noticed the same type of white stuff growing on one of my Glo fish's eye so apparently the Primafix isn't what I need.

We had a similar situation with my 10 gal tank several months ago and it completely wiped out the tank. After the last fish passed I gave up and broke the tank completely down scrubbed it and set it up again. I didn't add any fish to it until I had tested the water and everything had leveled out and the water was ready. Now we have a bug-eyed goldfish in it and nothing else will survive with him but that's a whole other thread.

I really don't want to lose the battle in my 20 gal tank as I've had it successfully set up with these fish for a good while and am happy with it and up until a couple of weeks ago, it was happy with my care.

What have I done, what is wrong with my fish and how can I fix it without hurting any of my fish or plants? I'm a novice when it comes to terminology and such so please be as specific and simplistic as possible when responding.

Thanks in advance for all your help.