I kept 5 RBP and 3 (or 4, it's been awhile) pleco in a 125 and they were happy as clams. Do your research and you'll find that raising happy RBP has more to do with lighting, temperature, and feeding than it does tank size. RBP don't need hiding spaces, they're not as territorial and aggressive to each other like the "same species" problems we have with other monster fish. Just my 2 cents, your 90 could house 3 of them just fine if you do your homework first. I would do a shoal of 10 minimum in a 300, and I've seen 10 in a 200.