Another Pleco died last night

Question
I set up a 20 gal tank about 6 weeks ago. I let it cycle with used gravel and decorations from the previous owner of the tank. I am using a power filter. I let it cycle for about 2 weeks adding food. After testing the water I purchased 4 zebra danios and 4 platies and a pleco. This first pleco died while still in the bag. I purchased another one and it died after 3-4 days. I had inquired about the temp and was told as long as the water stays around 70 they should be fine. Well, one of the danios passed and I replaced it and again got another pleco. Shortly after I purchased 2 mollies and a fiddler crab. After a few days one of the mollies started acting real strange. It started racing up and down one side of the tank along the same corner. After 2 days of this it passed, and a day later the other one as well. After another 2 days one of my original platies passed, and another 2 days later another was gone and one had disappeared. Besides some artificial plants I also have this reef looking thing. Quite large with lots of hiding places and the missing platy was trapped under it-dead.And yet another pleco died. So now I had the 3 original zebra danios and 1 of the original platies left. The fiddler crab was nowhere to be found, although I made sure there was nothing it could have climbed up on and out of the tank. So now I spend a lot of time researching on the www. The only thing I could come up with is that my water is too cold for these fish and promptly got a heater. At this point I did about a 25% water change and slowly added not cold but warm water to gradually increase the temp. I use the treatment for chlorine. I forgot to mention I had done a water change previously, about 20%. I check the water periodically and the only "bad" thing is that my water is quite hard. PH always checks ideal and ammonia hardly registers, nitrite is next to none and nitrate registers in the safe zone. I also forgot to mention that some of the platies seem to be always hiding in the reef or around the plants. Well, when the water warmed up all my fishes started swimming around in the tank, no more hiding. that was a relief. So then I started constantly checking the temp until it got around 78 and then it went above. So I constantly made adjustments. It seems to have leveled out to between 76 and 78 and that's where I left it. I got 2 more mollies, and 3 more platies to keep my only original survivor company. I also replaced the one zebra danio and got another fiddler and another pleco. The next day I suddenly had 2 fiddlers in the tank (?). I saw them both for several days now the first one is in hiding again and I can't find him again. The zebras frolic a lot, the platies enjoy just swimming around (only one of the new ones, the littlest one, kept his top fin down and seemed to wanna hide all the time, but after a few days he seemed fine), and the mollies are fine, although one of them seems to be a bit of a bully, chasing blue constantly, the original platy and sole survivor of his kind. I check on Oscar (the pleco) constantly, always looking for him to make sure he's okay. I also supplement with algae wafer and he seemed to enjoy the chunk of cucumber I put in the tank a few days ago. Yesterday I chopped up a little organic spinach and they nibbled on that. I have also added some bulbs and hope to have some live plants in my tank soon. When I got the heater I also purchased a bubble curtain and the danios and mollies seem to rather enjoy that as well. Sooo-maybe all the other fish froze to death? Poor babies, I felt so bad. And Oscar----well, I found him this afternoon, he was gone. One thing I noticed about all of them, the fins seemed to be frayed, on the tail and on the back. I observed this on Oscar last night and got worried. He had started eating the algae off the reef and today he's gone. None of them made it a whole week. What could be the cause? Any help would be greatly appreciated.

Answer
Christel,
The first problem I see is, you should have cleaned out the used gravel and decorations before putting them in. Here's why: if there was a problem with them before...you have now just introduced that problem to your new tank. Here's why I don't cycle using food. You must REMOVE uneaten food as it will contaminate the water. As far as the first pleco dying, that is a "store issue", the fish was obviously sick when you bought him. Assuming these fish all came from the same store, I suggest  you find a new pet store to buy your fish at! Your Pleco is a tropical fish and although they could survive in 70 degree water, they most certainly won't thrive, nor will they be happy. Set your heater at a minimum 76-80 degrees. This should keep All your fish content. I must ask you why would you continually buy fish for this tank, when they die shortly after being introduced to it? The problem is obviously THE TANK!! I would take the remaining fish out and put them in a hospital tank. Completely clean the first tank, gravel, plants, decorations, filter and heater(using ONLY hot water-NEVER use soap of any kind) Now you must re-cycle the entire tank. Please do not use food this time. Fill your tank with conditioned water, add your filter and heater and just leave it for 2 weeks. Don't even test the water, that comes after the 2 week cycling. When cycling is complete you can introduce your fish to their new tank. It is really important to understand, when you  bring home a new fish, it must first spend a minimum of 2 weeks in the hospital tank. The reason you quarantine new fish is because any sickness the fish may have come with, will show up in those 14 days. This keeps your community tank safe from illness or death. I hope this helps.