I've played D & D for over 30 years and can't think of one single thing! Arggghhh! My creativity is blotted out because I am at work, that's it... I'll think of something...
I tried to match aquarium stuff with stuff I remember from the 2nd Ed DM's guide and this is the only one I could come up with for now...
Bag of holding (plastic bags)
description: these rods come in many and varied shapes and sizes. each of these wondrous small rods offers your tank the following bonuses:
+1 against nitrates/ammonia
+1 against algae out breaks
+1 to O2 levels available in the water column
special feature: 10 or more rods of water purification in your tank offer an overall +5 bonus for dazzling onlookers.