Just my opinion and for what its worth:
Assuming your "turbo snails" are Mexican Turbo Snails, 4 is far too many in a 20g IMO, unless you have an awful HA problem. The most I've ever had in my 47g is 2 and then only as long as it took for them to eat the HA in the tank.
Both the turbos and the astreas are algae eaters and, with the peppermint shrimp, should be regarded as part of a CUC (Clean Up Crew).
Do you have a terrible algae problem?
Personally, I would feed small pieces of raw table shrimp or raw fish to your peppermints, and then, maybe 1-2x per week.
If I saw 10-15 pellets twice a day (20-30 daily) go into my 47g tank, I would have an aneurysm. This sounds like a likely source of your phosphates.
I have 15 nassarius snails, 10 astreas, 3 peppermints, 2 cleaner shrimp, 10 nerite, a couple cerith snails, and 3 fish in my tank and I only feed "a very small amount" every other day, and sometimes I think I feed a bit too much.
The first thing is to quit putting phosphates into the tank, then worry about getting what is in there back out.
Macroalgae, phosban, chemi pure elite and other methods/products can help reduce what is in your tank, but first, quit putting it in!
Also, a 3 week old tank?!? Has it established the cycle in 3 weeks??
130w on a 20g sounds like a lot of light too...these are PC lights, correct?