I've used under gravel filters for years, and they work great if you follow some common sense rules. Especially if you're running an under gravel filter in a small aquarium like the seven gallon, don't put in too many fish. A half dozen guppies should do fine. You can feed them a couple times a day, but make sure they eat everything in a couple minutes. If they ever appear to not be hungry, the aquarium probably has too much ammonia and is overdue for a partial water change. Live plants are a big help, because they convert some of the ammonia to plant food. They also use up the nutrients that would otherwise grow algae. If you do wind up with too much algae, try floating some hornwort on top, and use a lower wattage light. Above all, make sure you change about 15 percent of the water every week, and vacuum about half the crud off the gravel.
I hope some of these ideas help. Please let me know how it all works out.
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