Trying to figure out what's causing a string of fish deaths

This is really starting to bug me. A year ago, we started out with a 30 gal. aquarium. Did everything by the book. Had minimal initial losses. All the fish seemed healthy and happy. Water levels were normal. Then, about eight months ago, things started to go wonky. It started with a beta that we'd had for about two and a half months. One day he got lethargic, hid at the bottom of the tank under a rock most of the time, and when he came out, the other fish wouldn't get near him. Less than a week later, he died. There hadn't been any new additions to the tank in over a month and all of the chemical levels (ammonia, nitrites, nitrates, etc) tested well within acceptable (no ammonia, no nitrites, under 20 nitrates). But after that, every few weeks, another fish dies with similar symptoms. The latest was my last adult molly. She hung out at the bottom of the tank, barely seeming to breathe at all. None of the other fish would go near her. She seemed to want to eat, but when I fed and she tried to come up to eat, she seemed "unbalanced" if that makes sense. Like she had very little control over how she swam, almost flopping around in the water, and she couldn't make it up even to the mid-level of the tank before flopping back to the bottom. She had no visible damage and looks normal in every way at first, though for the last day and a half, she bent more and more in the middle, with her tail curling towards her head until, at the end, she was almost bent in half. Some of the fish that have gone through this have developed what looks like a whitish pimple or small pustule somewhere on their bodies, but others haven't. There's nothing obviously wrong around the gill area, fins look normal and healthy, everything. No new fish have been added in over 5 months and water levels are still absolutely normal. I do weekly partial water changes. I can't find ANYTHING at all amiss except for the fact that one by one the fish keep dying! Sometimes it will average out to one a week, but sometimes we'll go a full month or more without losing a fish, only to have this whatever it is resurface, even though nothing has changed. Someone please help me figure out what's going on.