55 Gallon Aquarium Stocking?

Well, what do you like in fish? Do you want low maintenance? Do you like communities or one or two larger fish?

If you haven't set up the tank yet, you'll want to wash it and the gravel in how water with vinegar, you can use salt too, let it sit overnight with that solution, then thoroughly rinse everything. This guarantees killing nasty bacterias better than hot water, which can actually cause some nasties to bloom instead or just foul things up if it kills them but wont rinse off.

You could do schools of several different types of fish from barbs, rasboras, tetras and danios to livebearers(fifteen to twenty depending what you go for, be sure you check their space needs!). A few of the smaller cichlids too.

You could do a pair of fancy goldfish.

You could do a dozen barbs or tetras or danios(or six of each) and a single red-tailed shark.

You could do one Oscar, it's a little tight, but 55 gallons is the minimum for one.

Rams and tetras or barbs are neat.

There are probably hundreds of options, but we need more on where to start, what type of fish, colors, sizes, maintenance levels etc.

Check very carefully for leaks.

While the tank is still outdoors, fill it with water and watch for a few days to see if water is seeping, running, or oozing out anywhere, or if the water level is dropping faster than evaporation can account for.

I think you're better off getting a larger filter, or repairing the old one, than using small filters on that tank.

You could do a community tank with schooling fish, a chiclid tank, a rainbow fish tank, a discus tank (well, maybe...), a livebearer tank or a mixture of several choices.

Best to pick out some fish and then we can create a stock around those species.

Depends do you want a community tank?

Could have a lof of small fish in there like tetras gouramis barbs or cichlids like the parrots Or if its tall angel fish many choices of fish except the larger cichlids oscars red devils and so on they need alot more space 75g+