well that 90 Gal tank will be good for about 10 months, if he grows like mine are... Currently i have 6 TSN, 2 4" had them about 14 days now and already they have eaten 8 of my 1 1/2 inch Neon Tetra's, those to are currently housed in a 55gal tank waiting to be put into a larger tank, then i have 2 8"-10" in my 120gal tank, there only about 7 months old now, and i have 2 more in 200gal, there about 14 months old and there at least 12"-14"... TSN grow extremely fast and will eat just about anything that they can put in there mouths, these fish require large amounts of space, other wise they will tend to try to get out of there tanks.... As a Aquarium Hobbyist, if you can't get this guys a bigger tank, and then a bigger one after that, Please Please try to locate a Hobbyist that can care for this guy the way he needs to be treated... The easiest way i have found to determine there tank requirements is for every inch of fish they need about 10gal's of tank... Of course i could be wrong but it is working for me, and there great fish to watch.... Good luck