Do you mind posting a full tank shot? An Evo is a small tank for that much CUC, especially the sea hare and the pin cushion.
Recommendation is to graduate from API tests to higher quality like Salifert (my fav), Red Sea or the dreaded Hanna.
Pull the PhosGuard (it's doing too much) and try feeding more to get those nitrates up. Reduce the water changes. If NO3 and PO4 are staying stubbornly low, but your elements are dropping, try dosing something like All For Reef inbetween less frequent water changes.
To help your corals and perhaps your tank all around, you might try spiking some bacterial diversity in the form of Fiji Mud. You can just get a quarter to half teaspoon, mixed it up in tank water and pour it into the tank. Once a week till the tank starts looking like it's improving. The good stuff will settle into your rocks/sand and start seeding new life that will overall benefit the tank--SPS corals tend to appreciate the mud. The old Walt Smith Fiji Mud is no longer available, but mud from Fiji can still be bought from AquaForest. Note that there are magnetic particles in the mud collected from that part of Fijian waters, so best to remove them before dumping into the tank. See my post about how I do it:
Big batch of baby brine shrimp, but the wrasses hit the sand today when I cleaned the glass. Oh well, Fuego the firefish is enjoying it at least. https://photos.app.goo.gl/39RhLn5fmYaA18oDA
www.reef2reef.com
Note: an Evo is much too small of a tank for a melanurus. You'll have a bit of time while that guy grows, but he won't be happy about the cramped quarters when he gets 5 inches long.
Bristleworms are just ugly, but they put in good work. Watch out for Oenone fulgida worms, some Eunice worms and fireworms (naturally, also watch out for flatworms). Those are the bad guys.