Buy onions in that plastic net, it's usually yellow or orange, thus nicely visible.
Go to a store that sell aquariums with freshwater fish, especially tropical.
You can get frozen bloodworms or brine shrimp for food, or make your own. There's some youtube videos on making your own food for freshwater tropical fish.
We had just unfrozen a turkey; we used the contents of the bag (hear, liver, etc) boiled as a base in a blender. Add water, the cheapest shrimps (unfrozen) you can buy at the store, a box of fish flakes (for vitamins), lastly fish paste or "fake" crabmeat, and blend it. You can also all a few boiled eggs - fake crabmeat "sticks" are made with fish paste & eggs.
Blend well, add water to get a paste that runs like when you make pancakes.
Put waxed paper in a pan, and pour unto the waxed paper, put in freezer. A few hours later, take it out and break into pieces / chunks of about 1", and put all this in a Ziplock Freezer bag.
Our tropical freshwater fish LOVE it. Sometimes one will "guard" a chunk that sunk to the bottom from the other fish.
Now about the "net" that comes with the onions. This is to capture a bunch of homemade chunks, so that they don't float away every which way. Add a copper or lead weight, attach a fishing line if you want to retrieve it w/o diving, and place it.
If tropical freshwater fish love the homemade mix (including catfish-like species) then your "mix" will be a hit.
*** Much like feeding birds, though, you will create a habit. Some fish will only lay eggs when food is abundant; artificially adding food to an ecosystem might put it off balance.
