The only type of seahorses which can go in an 8g tank are dwarf seahorses, H. Zosterae. They are quite budget friendly, but the catch is that due to their size, 1 inch as adults, they have to be fed very small prey. Because of the quantity of food required- a lot, twice daily- brine shrimp are the only practical choice. Which means that you will need to hatch and enrich brine shrimp daily, bi-daily, or weekly, depending upon how much you hatch each time. While preparing the Artemia for your seahorses is by no means difficult, it is tedious. Their are many benefits to dwarf seahorses, however: because they are so small, you can safely keep 20 or 30 of them in an 8g aquarium, also, the fry can be raised in the tank with the parents with no extra effort on your part, and they breed prolifically, so you can start with two or three pairs and have several dozen a few months later. There is more to it than what I have mentioned here, because I just wanted to give you a quick idea of some of the advantages and disadvantages, but fusedjaw.com and seahorse.org both have excellent articles. Also, their are plenty of old threads where the basics are outlined. For larger species of seahorses, you would need at least 29g for a single pair, and you would also need to buy a chiller: that alone would cost you $400 easily, not to mention the expense of livestock, substrate, decorations, etc. Given that you appear to be looking for an economical way to please your girlfriend, I didn't think you wanted to spend so much money. Dwarf seahorses are considerably cheaper. You could easily do it in $100 or less if you went that direction. Hope this helps!