I use Eric Borneman's coral food recipe, calls for fish roe... 4tsp a night feeds 300g. So, you can imagine that it lasts quite a while. In a 20g tank it equates to about 7ml +- per day. I gotta experiment with it in such a small tank, and do some more math. It's impossible to wash under tap - or else you'de be getting rid of all the good stuff.
If you do feed mysis, brine, or other enriched frozen foods, do wash it under the tap. The excess liquids can cause phosphate problems. I feed my fish a variety of flake - always have, and have never had any complaints. lol!