Mass-effect – What does the button on Aquarium VI do

For a whopping 25,000 credits, the Aquarium VI is available for sale on the Citadel, at the Elkoss Combine Arsenal Supplies terminal (where you first find Barla Von) in the Presdium Commons.

In addition to breaking your piggy bank, this pricey item's description also indicates that it will automatically feed your fish:

With so many exotic speices of fish available in today's galactic market, it takes an expert to maintain an ideal aquarium. This VI automatically dispenses food, adjusts the temperature, pH, and algae content of the water, and even talks to the fish when you're not around, leaving you free to enjoy your aquatic friends without fear of killing them.

If 25,000 credits isn't expensive enough for your tastes, you can also head on down to the Alliance Requisitions terminal in the Normandy's cargo bay, where you can buy the same Elkoss Combine Aquarium VI for base price + a 2,500 credit markup.

For the most part, nope. No reason not to always have an Ammo power activated.

An exception might be if you are specced into, for example, the final rank of Incendiary Ammo and the explosive effect would be inconvenient for some reason (Perhaps you're trying to snipe a target without blowing up a strategically placed explosive barrel or something.) Those situations should be pretty incredibly rare though.

Ammo Powers will never leave you vulnerable, they will not cost you credits, will not damage your weapons, and will not kill your fish. They will allow you to do more damage, apply useful debuffs, and, with upgrades, carry more rounds.