OK, so I'm relatively new to the whole fish keeping, my parents had aquariums when I was growing up, but I just started on my own about 9 months ago and I'm still kinda muddling my way thru...

I started my 10 gallon tank with 10 neon tetras, 2 of which died the following day, but the other 8 had been doing fine until last week when 1 started swimming vertically and all of the sudden and then just died. yesterday I noticed that 3 of the 7 I have left are losing color on their red stripes, it slowly turning white in patches. I've looked everywhere on the net trying to figure out what's causing and it looks like Neon Tetra disease, but I don't know how they could suddenly aquire that after I've already had them so long. They are still schooling and no one is getting picked on, but they are gasping alot. They aren't going to the surface to gasp, just doing it as they swim around the middle of the tank.

A few things about my tank: It is overstocked. I didn't plan of that happening exactly, I'd gotten 7 pygmy cories to fill the tank out a bit figuring they were small so they shouldn't add to much bioload and I change 20% of my water every weekend. But after about a week I never saw the cories again and after about 4 months figured they must of died and were so small I'd missed it somehow. So I added a few panda cories, and of course then all the pygmies came out. That was 2 months ago, and I haven't added anything new since.

I have a sand substrate, but it has been a few months since I was actually able to give it a good cleaning. My assassin snails were so happy they decided to repopulate and some of the babies are so tiny the vacuum could pick up them up easily so I've just been sifting the sand and sucking out the water when the debris floats up.

I'd thought about transferring my panda cories to my son't 10 gallon with his betta, he has 3 pandas in there and I'm sure they would be happier, but if they are sick like my tetras I don't want his tank infected too.

I don't have a water test kit, but as soon as the weather lets up I'm off to get one. Any help would be appreciated....