5gal AND 2 gal into reef-feeder tanks

Hello!

I would like to hear everyones input on this matter:

I have a 5 gallon FW (HOB, powerhead, good lights for plants) and a 2 gallon hex tank with a UG filter that I am going to convert into feeder tanks for my 80 gallon TR. They would also serve as quarantine tanks as seen possible.

Ideas? How deep can one go with a UG filter? 5"-6" of big open gravel might hold some promise?

What can you all tell me about types of invertebrates that you know to breed well in captivity?

After I change the water my next step will be collect and ID some potential candidate species.

Some kind of small shrimp I'd imagine.

What about copepods or other "pods".

I just put a bunch of sand and rocks from the beach in the 5gal Couple crabs.

During the upcoming neap tide I will go way low on the beach and dig with a shovel. Must be some yummy critters in there to grow... no?