Take advantage of the season and pick up a zucchini at a market. Cut it into coins, then put wax paper between the slices and freeze it. The freezing will break its structure, allowing herbivorous fish to get to it. The rind holds it together, but has to be fished out when they're done. You may be amazed at how they go for that once they figure it out. It's dirt cheap (I grow them for this) and very nutritional, and if you choose the size of the vegetable carefully, causes no mess. You want a middling sized one.
I also use a local coastal product - dulse, a purple/red seaweed my fish love. I also use all manner of algae wafers, though with the zucchini coins, a bag lasts probably beyond it's peak nutrition. I haven't bought wafers for a few years, but the bag still has some in it.