In the 125 it will probably be better to start with 3-5 depending on their size. If you plan on upgrading to the 300 in the future you can always add more. I plan on upgrading my current armatus tank asap, my cousin builds fish tanks for a living, but I won't see/talk to him until xmas dinner.
As Chicx said, a tat or school of scombs would be fine in a 300. An armatus would need the largest footprint possible. These fish do require a little extra care in my opinion, their setup needs to be to their liking in order to thrive. Prestine water quality is a must, with good flow and airation, the exact flow amount is debaitable between owners.
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