90 gallons!
So many options.
Do you know what type of water you are dealing with: kH, gH, pH, that would be a good starting point, as its always best to have fish that match your tap water?
Do you prefer a few large fish, or would you prefer a lot of little fish? (Personally, I'm a big proponent of lots of little fish, as you get to see more of their natural behaviors that way.)
If you have hard, alkaline water, a group of African cichlids from any of the Rift Valley Lakes would be great... no plants necessary - Cichlids are notorious for ripping up plants. These are of the more aggressive nature, but if kept together, in the right numbers and in the right aquascape - plenty of rocks to give each a private little territory to defend and retreat to... they can be a stunning display. African cichlids are among the most active and colorful of all freshwater fish. Lots of colors to choose from, but this may not be your speed.
Another option for hard, alkaline water would be livebearers and rainbowfish. Livebearers don't have to mean lots of fry, if you just keep males, no breeding will take place. Lots of folks can't keep many of the varieties of rainbowfish due to the length of their tanks not being long enough. I assume your tank to be either 5 or 6 feet long, and that will work for nearly any of the rainbowfish that I can think of. Again, lots of color options for both the livebearers and rainbowfish.
Moving towards the soft and acidic water, South American biotopes offer some lovely choices for community tanks: corydoras species, a massive variety of smaller tetras (many are very peaceful, especially if kept in sufficient numbers, though there are a few that are a little more snarky), even mixing in some cichlids (not all are aggressive - rams, apistogrammas, and even a lone angelfish could work out). Mix in a BN pleco or similar and there you go.
Another option from Asia would be some rasboras (harlequins, espei or hengeli), danios, pearl gourami, etc.
You could look into barbs also, as long as you are careful, as some are a bit more aggressive than others. Cherry barbs, five banded barbs (not to be confused with tiger barbs), odessa barbs, and redline torpedo barbs are all peaceful enough to coexist easily.
Basically, your tank doesn't really limit you very much at all. Your tap water and personal taste will be what drive your final plans. And we'll be happy to point you in the right direction. Living in the US, you might want to check out aquabid as well as a few other places that we can discuss later for sourcing some of the less common options. Buying directly from breeders is more expensive, but you are usually going to get much healthier fish than at the big box stores, though I've done both.
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