Corals not opening as much after water change, please advise.


Reefing newb
Hello, I have a 75 gallon tank with two clowns and a firefish. When I first started the tank in June, I was on top of the maintenance, but then I got really busy and missed a couple water changes, my corals were doing well, opening up, and retracting every night, and my parameters seemed good, so I was doing everything except a water change, I was scraping algae off the glass, cleaning the skimmer cup, cleaning the mat in the sump, and swapping out the carbon and phoslock in the sump. Everything seemed great, but then I started getting taken over by brown algae. My tank seemed to always have a little higher phosphates which is why I was using the phoslock, but it was getting out of hand. The fish and corals were doing wonderful at this time. So because of the algae, I finally made time to do a water change, about 12 gallons. I vacuumed the algae off the top layer of sand, I did not stir the sand, only the top. I took a turkey blaster and sucked up some off the rocks, but most of it ended up floating in the water. When I was done the water had a lot of stuff floating around in it. After I replaced the water, it filtered itself and the water was clear the next morning. The next day when I fed the fish, the corals were not opening as much as before. My Frogspawn opens but is not as big or as inflated, my flowerpot opens, but does not extend as far as before, and my pulsing xenia opens but looks kind if shriveled, not these big open pulsing polyps, but smaller and not pulsing as much. I tested my parameters and it seems everything is better than ever.
Ammonia is 0, or very close to it.
Nitrites is 0
Nitrates is 0
PH is about 8.0
Calcium is 440
KH is 161.1
Salinity is 1.025
Phosphates is between 0.0 and .25
At first I figured I stressed them out because of stirring all the settled crap up in the tank. But 6 days later, they don't seem to be getting any better or worse. Is it possible they were used to that old water and when I did a water change after missing 2 that it really stressed them out? I'm just not sure what to do, and what I did. Please help.
It is a theory that soft corals and LPS prefer some nitrates in the water. That could be a part of it. Are you sure nothing else is bothering them or nothing else has changed?
I don't think 12g would have made much of a difference, even though what kyle says is true. We're only talking 15%.
Your parameters looks good, but what are you using to test for phosphate? That is a HUGE range and .25 is really high. It should be below .1 and ideally around .03.

I'd give it a little big longer and go with the fact that you stirred up a lot of junk.
I'd also try to keep up with those 10-15% water changes.
I'm really at a loss, the only thing I have added is 10 turbos to help get the algae under control. The previous times I did the water changes, they did not have problems with it. I have used the same salt, Instant Ocean. I'm trying to finish it up before switching to reef crystals. I let it mix overnight, matched temp and salinity. I hope it's due to the fact I missed some changes and stirred up a lot, but if that's the cause, this reaction is enough to make me not miss any in the future. I am worried though because I should do another water change in a week, and I don't want to further stress things out, but on the bright side, my fish seem content.
Don't worry about water changes. They are a good thing. The only time it can be bad (when otherwise done correctly) is if you have a bad batch of salt, which is really uncommon.
The tiniest amount of some toxin in the mix will make the corals look bad. Clean containers with lids is important. I do know someone who cleaned their glass with Windex and watched corals shrivel. Bad idea. Run lots of carbon with a strong filter like an HOB are even a canister filter (temporarily). If there is some toxin in the water the carbon may be able to get it out.
What are you testing phosphates with? I was using Salifert color testers and it always said my levels were good, but was having similar problems as you described. Got a Hanna digital phosphate checker which showed my levels were very high, I was totally shocked at difference between the 2 testers I run a reactor with GFO now which helps a lot, I do 25% water change every 2 weeks and have a bag of Carbon in the sump next to the discharge from the skimmer. My levels are coming down every week now and the corals are looking great. My tank is 90 gallon and I have a 50 gallon sump, my total water volume is approx. 100-110 gallons after displacement of rock and equipment. so my water changes are approx. 25 gallons every 2 weeks right now, once my levels are at 0-0.03ppm I may go every3 weeks.
Phosphate checkers are kinda worthless. They either show 0 or very low because if you have high phosphates and algae, the algae is using it fast enough to give a false reading. I won't ever invest in one. If you have an algae problem, I would make sure you're using RO/DI from clean filters, rinse your frozen foods in RO/DI before feeding, avoid flakes/pellets, and try to get some chaeto or macro in your tank/sump. And also, more frequent water changes will lower the phosphates each time if you're using RO/DI.
I never mentioned that I do have a RO/DI system, I guess I just assume people that have salt water tanks know that RO/DI is a must. I also agree with you regarding chaeto or macro in the sump. The Hanna phosphate checker is a good tool to have if your interested in your true levels though. Having said that I have a good friend who is my mentor on reef keeping and he never checks his phosphate levels he just goes by how his corals look and his 180 gallon reef is beautiful. I'll try and get some pictures of it and post them.
Have a Great holiday season everyone