Welcome to our forum Bribree.
I would definitely return that molly to the pet shop. I love my mollies but I fully realize that some of them can be pure devils in a tank.
Do not bring home any new fish though. Let me explain something rather simple but that your boyfriend and the pet shop have failed to tell you. All fish produce some traces of ammonia by simply passing water through their gills. Ammonia is also produced by fish wastes decaying and by any uneaten food decaying. Unfortunately fish are quite sensitive to ammonia levels in their water. Some fish, such as the common plecos, are more sensitive than others to ammonia poisoning. Your pleco could not have starved in under a week, almost any adult fish in one of our tanks can easily go 2 weeks without food and show no ill effects. OK, so where are you? You are smack dab in the middle of a fish-in cycle if you have a filter in that 10 gallon. The problem with adding any fish at all right now is that it means you will accumulate ammonia faster than if you do not ad them. I know all of this sounds pretty dire but it is really quite easy to deal with. What you do is test for ammonia in your water using a liquid type test kit, many of us here use the API freshwater master test kit but any kit that uses the little test tubes and liquid reagents will give decent results. Whenever you see any traces of ammonia with the test, you do a large, in my case over 90% is my own personal norm but it need not really be that big. The idea is to dilute the tank water to a point where you can no longer detect ammonia. Over time, often it takes several weeks, you will find that ammonia just never shows up any more. That is because bacteria that are naturally present in small numbers in your water supply have reproduced in your filter and now are processing all of the ammonia to become nitrites as fast as the fish can produce it. The bacteria normally live mostly in a film on surfaces but there are small traces of them in the water supply. The water changes do not really remove much of these bacteria because, once again, they are concentrated in the filter, not in the water. An unfortunate circumstance is that nitrites are also poisonous to our fish. It affects them in much the same way that carbon monoxide affects mammals. Even when there is plenty of oxygen in the tank water, they are unable to absorb it effectively because the nitrites have taken up the position on their hemoglobin that should be carrying the oxygen. Other bacteria exist that will process that nitrite into nitrates. Nitrates are almost harmless compared to nitrites or ammonia so a weekly small water change is all we need to do to control them once the bacterial colonies are well established.
For details on the cycling process with fish in the tank, please follow the link in my signature area that is marked as fish-in cycling. It is well written and will give you more detail that may help you in your present situation.
