why are you even concerned about 20 ppm
that is essentially nothing.
fish can tolerate much, much higher levels than that and a lot of the info I have read actually leaves the impression that there is still debate whether nitrate at the levels we encounter is of any concern at all.
of nitrate, pffff, not even worth a second thought.
WHAT you should be concerned about is that indicates that the ammonia and nitrite also spiked, but most likely your system is established enough that you never notice.
Remember, nitrate is an indicator of overall nutrients entering the system and the subsequent breakdown of them.
The best diet for any critter is a varied one, so why not both feed pellets and brine shrimp?