Thanks for all the help, Tab & Froggy.
Yeah I do vacuum the gravel rather well each time I do a wc. But, it's funny because it seems like there is always quite a lot of poo/uneaten food (so I may not get it all) and I've made sure to only feed them once a day and sometimes just every other day. The guppies seem to rarely come to the surface to eat flakes, but since they aren't dead yet, they must eat at sometime. I put a 1/4 of an algae wafer in usually every other night for them and don't typically see any left over from it the next morning. I've seen both the guppies and the cory all nip at the wafers.
Yeah, I've had Blue for about 2 years now, but I just rinsed out everything in his tank a few weeks ago because I accidentally left his light (and the 10g light) on all day while I was at work. The 10g water was okay after it cooled down- both got to about 80. But, Blue's water was really murky, where as before it was clear. I did 3 75% wc's in the course of a week but it made no difference, so I rinsed it all out. He normally gets a wc once a week, keeps his water clear- he poo's a lot.
I'll check my tap water. It's city water so who knows whats in it.
Alright, I'll start thinking about that new tank soon then.
Thanks again for all the great advice, I really appreciate it!