I've had a few mini crashes on my old tank and even a very large one. I've personally been on the side of trying to solve the issues without chemical additives. After my worst tank crash (hurricane Florence took out power for 14 days) I had a ton of diatoms, cyano; it looked super scary after that. I was able to get everything on the mend without additives and just keeping to the usual schedule without a 100% water change or anything like that.
The most important things I think I've found for dealing with these is bullet listed here; hopefully it helps.
- Ensure you have very healthy chaeto & refugium with plenty of light and water flow. These should out compete pretty much everything except your corals and fish.
- Very healthy high quality live rock is a huge benefit -> buy a good 5+ pound premium rock (the stuff thats like $15/lb). Stick it in your refugium; it should come with good critters. My running theory is that the baseline ecosystem that comes with these rocks greatly helps the overall health and spreads throughout the aquarium.
- Maybe bump the waterflow on your display tank and put some filter socks on for a while (if you don't already). It tends to prefer slower areas of the tank and if it detaches, you can catch it in your socks and clean those out instead of it recirculating in your system.
- Consider Natural Eaters & Things that disturb it. Sandfighting conchs or any other sifter for the sand, sea urchins are great; they get everywhere, eat pretty much anything like this and disrupt a bunch of stuff you don't usually like (they do eat corraline; but ive found in a good tank the corraline grows faster than a small urchin can eat it), emerald crabs can sometimes get rid of it.
- I think I read you are using RO/DI water; keep that up for sure
- Use more natural foods such as frozen mysis or brine shrimp (even live ones) - shoot I even blasted my whole tank with that stuff and way over-fed; my tank compensated by having amazing chaeto growth and more pods than I could handle; so I tend to add a few more hungry fish to help balance it all out which produced more detritus; but the base ecosystem had brittle stars and bristle worms and all sorts of stuff that loved to eat that stuff and the flow was pretty high on the tank.
- Much of the flow of my healthier tanks seems to come from the movement of water from display to sump and back, the additional heads are just to ensure the whole tank can get movement and produce detritus lift.
I don't know if any of that is helpful; but doing those things has been working for me; your experience may differ.
