Dying fish...help!?

We have a 10 gallon tank with 4 tiger barbs, 2 tetras, and a sucker fish. We woke up this morning and 1 tiger barb and 1 tetra have completely disappeared. Eaten? 2 tiger barbs and the other tetra are dead. The other tiger barb is dying. The sucker fish seems okay, but its hard to tell. Yesterday, they seemed ill, so we changed the water and tested the water (it tested okay, but we only have the strip testers which I found out a couple of days ago don't do a very good job). The temperature is right at 75 degrees. we can't figure out what's wrong but we took the two remaining fish out of the tank to try to save them. We REALLY want to save the sucker fish, if at all possible because we've grown attached to him (His name is Gary). Thanks for the help!