Mass-effect – treat the fish to too much food

Being below weight capacity proportionally increases power recharge speed. Being above it decreases it. Carrying less guns reduces your weight (thereby increasing recharge speed). It does not affect the amount of ammo individual weapons hold (mods do).

The wiki weapons entry doesn't currently have individual weights for weapons, but likely that information will be added soon. In the meantime on the weapon equip screen as you add/remove weapons it will give you a percentage penalty/bonus to recharge rate. This can give you a base idea, but ultimately if you use a lot of powers you'll want to equip as few weapons as possible and put some points into increased carrying capacity.

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.