Over the last 5 weeks (most of them in the last 3 weeks), I have lost 3x 5.5"-6.5" discus and 1x 8" L418 pleco. I have seen no symptoms at all. I just come in and they are suddenly dead. They are all eating well, looking healthy, good weights, gilling normally and not a mark on them. I've dealt with discus and internal parasites before, but these discus show none of those signs. The pleco has been looking healthy too, with nothing but a nice flat gut (not sunken or bloated). The water parameters have been good with no ammonia, no nitrites, and only a small amount of nitrates. Its a 55 gallon tank with tons of aeration and filtration including two large sponge filters, two large marine land HOB filters and an FX5. I will be the first to admit the tank is over crowded (everyone has grown), and I'm in process of moving some of them out. However, there are TONS of hiding places, and the water quality has been good. Temp is about 79-80 and pH is about 6.8. I'm at a total loss as to what is killing my fish. 
Oh, I have noticed that sometimes there are a small amount air bubbles that come out of the gravel when I gravel vacuum. Could this be a toxic gas buildup? Could this be killing my fish? But why only one fish at a time then?
Oh, I have noticed that sometimes there are a small amount air bubbles that come out of the gravel when I gravel vacuum. Could this be a toxic gas buildup? Could this be killing my fish? But why only one fish at a time then?
What are the symptoms of liver flukes? What I'm finding online and what's been happen don't line up, who knows how they affect different fish. I'll treat with prazipro, which I do prophylactically every 4-6 months anyways.