I'm assuming you mean "dwarf" seahorses.
I have used a 2.5g tank but my preferred tank size is 5g and you can have about 20 in there no problem.
I don't know the Tetra line at all but I'm assuming it has a filter of some kind so you have to be sure to protect the intake so the dwarfs themselves, or even just their tails can't get caught up in it.
Personally these PnP tanks cost too much for what you get, especially when I can buy a small tank for around $10 or less.
I put a few live rock pieces in for biological filter and use open ended air line tubing for water circulation and plastic decor for hitching.
Because they only get fed on enriched ongrown bbs and the enrichment doesn't last long, I remove the left over food between feedings using a Marina i25 internal filter on a timer. (again, protect the intake portion but not with anything that will stop the bbs from passing through to the filter)
The timer allows for the filter to work about 2 hours and then shuts off and is ready for the new feeding.
It would be wise to start the brine shrimp before getting the dwarfs so you have the methodology down pat first.
