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Well, have I got a lot of questions, seeing as I got this 50 gal. Tank that needs a lot of work, and all Iâ??ve had before this is a 10 gal. Tank, so here goes:

When I got the 50 in July it had no light, no heater, really nasty h.o.b filter, a 10â?

You might as well change out all of the gravel while you're waiting for the tank to cycle. By the time the sycling is complete, it'll be good and ready for fishes.
If you do it in half measures, it will just take even longer with no appreciable difference in results.

That's where the problem starts. This tank was running at my Dad's apartment for about 2 years. He had to go into a nursing home, and someone had to take in his tank and fish. This was a sudden thing, and the tank was down for only 2 hours. It has been running with the fish in it for about 3 months, and if I tear it down to redo the gravel the only tank I have to put the fish in is a 10 gal. This seems really small for the 3 pl*cos, especially for a month.

That's why I'm trying to find out if I can change the gravel part at a time.

You can change all the gravel in a few hours the fish wont suffer. Just put them in a large container with most of the water fom the tank, put the filters and heaters on, and i would keep it in the dark with a lid.

I would choose either sand or gravel as it makes life easier.

Didn't you say you had a biowheel, there should be a pretty good colony of bacteria in there.