A 90 will be too small for the Fahaka. They can get quite big quite quick. I was in the same position as you last month. A Fahaka will need 120+ gallons when hes an adult. You have a couple options, but id suggest checking out the pufferpedia over on ************** if your dead set on a puffer.
I ended up getting a Monotrete Abei puffer. Only gets about 5 inches max which works perfect in my 55g which is a huge home for him.
Other options are the Brown puffer, Hairy puffer, Palembang puffer, small colony of Salivators, or maybe a small colony of SAPs? Others will have better info on the smaller guys, I dont know much about keeping multiple small puffers together. I do know you can only get away with one mail and there has to be alot of plants and hiding spots though.