Wow, that's a lot of fish for a 10 gallon tank. Like most aquarists, there are times when I have kept a lot of fish in a tank for one reason or another, but I always counter that with vigilant monitoring of the water paramters, frequent water changes, and adequate filtration.
Until your tank is fully cycled, it's not a good idea to stock your tank so heavily. That's often the downfall of a lot of set-ups.
When you switched over to the power filter, did you keep the undergravel filter running until the power filter was biologically active? If you turned off your undergravel filter before your power filter was biologically mature, your tank would have undergone a mini-cycle (ammonia and nitrite spikes) which could account for some of your losses. If you couple that with the addition of more fish sooner than you should have, the bioload would have been too much for your new filter to handle. You can speed up your cycle by adding a biologically active sponge from an establised aquarium filter to your power filter. Or you can add a cup of gravel from an establised aquarium to your 10 gallon.
For an overstocked tank, I would do weekly water changes of 25-30%. This may seem like a lot, but for a tank which has a lot of fish, it's just something that needs to be done. I would also be extra careful about how much you feed. I feed once a day , maybe twice as the absolute maximum, with a day of fasting every week (Sunday). Do you have live plants? I find live plants also eases the burden of a heavily stocked tank.