You placed quite a few fish in an apparently brand new tank. It could be related to your tank cycling. Have you tested your water parameters? (ammonia, nitrite, nitrate and ph). If not, it is imperative that you do so, either by taking some water to a fish store that has a liquid test kit, or buying your own liquid master test kit. Without testing the water you cannot eliminate it as the cause of your deaths.
The red pimple could be from infection (often due to water quality issues) or the fish picking on each other, but if fighting caused your fishes deaths you would have seen some injuries.
Your tank is pretty overstocked, even if it was established, and is newly set up and possibly not cycled completely. Between the two that is probably the cause of your deaths. 55 gallons is not very big considering you have added some fish that you stated are fairly large, and some that will eventually grow to a decent size.
Also, I'm not sure how much you feed, but overfeeding will foul the water pretty quickly.
Btw what kind of "shark" and "algae eaters" did you get? These common names refer to several different species, some of which can be aggressive or not compatible with your other fish, and some of which grow much too large for your aquarium.
If you want to try a website that is completely devoted to fishkeeping, I also frequent aquariacentral.com, and you would get a lot more advice there. There are not too many people on pets.ca that are fish enthusiasts. If you were looking for cat or dog advice it would be a completely different story.
