I Think that nearly every bit of advise that has been given to you is very close to the money.
In my opinion, and thats all it is, is a combination of a couple of easy fix things.
1. Water movement my tank turns over about 10 times its volume an hour that works.
2. Has to be a lighting issue if problems started after light change.
3. Diatoms on the substrate are a good indication of over feeding and Lighting.
Back the upgraded light back a couple hours and reduce feeding. Do this and gradually reduce this to see if the brown shit goes. Its a slow thing but, if its diatom (brown algea) then it will take a little while to be rid of.
I feed my corals and fish all together on tuesday friday sundays. No pumps or water movement. Leave for an hour and start all back up.
4. Maybe check your filter media. Could be causing something there. Bio balls are shit in my opinion. media could also be a little grubby causing your flow to be restricted.
Hope that helps.
Also stiring up your substrate can be a bad thing. It upsets the bacteria down there in the non oxygen zone

let us know how it goes ....we like to see how advise works out for people


Boss

you'll love it, i have the nano and it out performs bigger more powerful powerheads hands down