Add some Seachem Stability etc something like that to be safe.
And no, you can't put 2-3 of these in a 75Gal tank. Even 1 is a lot for a 75gal tank. It's not about filtration all the time. You could run 10 FX5s on a 75 gallon and have water blowing all over, filtered to the max, and you still wouldn't keep 7 full grown silver arowana in there. Filtration matters, yes, but size does as well, and this is a very aggressive fish that can get VERY large.