Seems to me Gaia likes sales and I like it when tank owners can find fish in the market place. My suggestion is, combine the two and bring back the old Phin Phang sales. A few sales here and there can be enough to ensure that there will always be mp listings for fish. ^^
Other fish suggestions posted in thread:
* Recolored fish and old fish in sales, as well as bundles of fish. Examples; A bundle of cat fish, a bundle of mermaids/mermen, bundle of starfish. (Combination of suggestions made by Cat Sapphire and Nyadriel)
* A new fish that while it grants gold, can also drop items.
Very similar to Enki or the giftbox droppers, in that it's a box, but I would like an new chance item. A cash fish that drops old Chance items, mid-to-low level in the price range, that would have a positive effect on the economy, and convince people to play BG in droves, as well as sink real currency into it. A different way to obtain, deflate, and increase supply of items. (Suggested by DarkRybrin)
* A cheaper fifth strike pack, with all the monsters that got cut from the fourth. more variety in tanks, people going to check those out could buy more fourth strikes. (Suggested by 2sarahgate)
* Being able to craft some kind(s) of aquarium fish through alchemy, perhaps with fish drops as components. (Suggested by Nekoofdeath)