guppy question

[pandafishowner]

I have a 5g tank setup right now on my pc desk. It has a filter (bought from wal-mart, supposed to be good for 2-5g tanks (uses filter cartridge replacements). I had 3 guppies when I put in the filter. Overnight 2 guppies died. I netted them and flushed them. My remaining guppy has been alone for over a month. The other night at Big Al's, I bought 2 new guppies. One was a male guppie with orange and black coloring on its tail, black polka dots on it's body. The other was a blue diamond guppie, also male. I replaced the filter cartridge as they need to be changed every 2-4 weeks, and tonight while I was out, the orange/black guppy died. Any idea what's wrong? I did an amonia test, there is none. My pH is perfect, same with nitrates and nitrites. Could it just be the filter causing problems? It's the type where I can adjust the power of it. I've got it fairly slow, not super slow, but not fast. The type of filter is made by for Small World tanks. I do water changes when I notice the tank is getting a tad bit murky. Last night I did about a 10% change, about a week ago I'd done around a 30-40% water change.