Tankmates ?

Well in a 125 he may show less aggression than in the 90, but from my experience they can be on the aggressive side. What was the plan to breed or just a wet pet? If you're not breeding I'd think a 125 would work well with other cichlids that are similar in size, but not shape/coloration.

Crytoheros, possibly since the size difference may keep the Argentina from battling to prove dominance. I think you could even get away with something like an Oscar, jd, green terror, jag, or even some oddballs or schooling fish if you don't want to gamble on the cichlids.

Ive kept bichir with cichlids and most of the times they will just ignore them. You have a lot of options with a 125g

I think he'll kill cryptotheros. I had 10 of them, down to 5 now had them about a year. I started them out in a 125 at 1-2 inches, and culled the weak ones/ugly ones, etc. I'm down to 5 now in a much larger tank, and that is the only thing that calmed them down -- moving them to a 540 with bigger fish. They were just nasty in my 125. They're very solitary fish, and they don't fight like normal fish (flaring gills, showing dominance, puffing up fins and swimming close). They just float close and freak out. Very violent stuff.

Although they grow very slow, they get big. An adult Argentea won't tolerate tankmates in a 125, I don't think. I think you'll end up with a wet pet, or a bigger tank. Because after you've waited 2 years for an Argentea to get 10-12 inches, you won't want to give him up.