water changes absolutely seem to exacerbate the problem. keep tank covered, light off until clear.
Honestly it appears you have typical Massachusetts water. The PH is raised because it extends the life of the pipes. Fairly common. I think your reading too much into things to keep those fish. Don't try to match some specific PH some website says your fish requires. Your fish are not in that need of specific exact care or that your too far from acceptable ranges. As far as FW goes, IMO conditioning your fish to our Massachusetts water within a acceptable realm is fine opposed to constant swings trying to perfect it by chasing numbers that will require a lot of chemistry work or investment.
Most FW are not coming from the wild at this point. My rasboras in theory should be in a PH of 5 > 6.4 , but they thrive just fine in my treated tap with a little acid buffer to bring it a little closer to 6.5/7.
How old is the tank, how many gallons, how many fish, what kind of fish, whats your filtration, whats your media, how often are you feeding, what, and how much are you feeding, how often are you doing water changes, and how much water are you changing out. What test kit(s) are you using. What kind of light(s), what wattage how long are they on, how many hours a day is the room bright with artificial or natural light would you say. What kind of plants, substrate, chemicals used? Too many questions?
Probably get a better direction after some background information is given. My 2 cents...
I am, only because I'm throwing out a direction to help you possibly save money, and RO, and PH kept mentally high lighting from your posts to me, that's all while leaving other things for others to address . Your scenario now is something i went through as my first big disaster as a fish keeper back in 97', and i was still naive teen, and took all this stupid advise, and everyone had a million opinions, and I remember going to Tropical Isle spending close to $200- on a uv, and pump, and tubing, and phosphate remover as phosphates were detected by their testing, blah blah.
Anyways years later knowing better, this type of scenario, there's probably a cheaper, easier way before spending the $, and If your going to spend $, target the issue(s).
Going out buying RO or Distilled gets old, and if you do buy a RO unit you should have your water lab tested to see if there's anything specific to target for removal, and maybe customize the RO to your specific needs, and since this is a reef forum I admit I'm curious if you have a reef tank, and if so, maybe the conversation about the RO should be more a RO/DI one. That was the questions i had, and if you do RO, depending on plants, and what-not you'll have choice's for remineralization additives to go through. If you do/add shrimp they themselves have their own needed values as an example for molting etc etc. Then from there there liquid ferts, because between the 2 if not measured out right is just adding too much of something, know what i mean. Much bigger separate conversation, but you may already know all this, just wasn't sure of how much you knew or not. A lot of just in case information
Have you considered maybe a canister filter opposed to what you have going on now? I don't know If my brain would let me go to bed with a sump over my tank haha. Have the sundadanio goblinus gone after the chilli's? Are the scarlets skittish or territorial or passive? Just curious, i keep scarlets with cardinals, rummys, amanos, and cories, but thinking about moving them to a mr aqua 12g with my chillis, and fires.
Was the cloudiness a white or green before? That can help determine if it was bacterial or algae in the timeline. Safetysorb is pretty inert, but I don't know how hands on you are in your tanks, but once that stuff breaks down, If It hasn't already It's just a cloudy goo mess. Just sounds like there's a lot that has gone down in the past few months possibly biologically, and the system equipment Is still new, and changing, and decent livestock per say given the filtration situation, ferts etc etc. IMO water changes would do you nothing but good, but in conjunction with monitoring your 3 basics of the nitrogen cycle til the system is biologically balanced, then tinker from there. The plants will love the water changes too. A UV is a good investment to have around, just run it on a small pump though, all about that dwell time. 9w UV with a range of 40gph>80gph would probably suffice for your size tank, then you can also use it for ICH weapon as well when needed.
I think your light schedule is fine, just not familiar with it's PAR. I have always had issue's personally with LED on planted tanks while not being co2 injected. So to end my ramble (had some beers), not sure if it helped any, or confused you more, I think you'll be fine with some water changes, and it'll probably fix your issue without getting too crazy.
What brand ro unit, and how many stages, and what are those stages do you know? Hair algae may not of been the ro units fault, but we don't have your history/background on it to determine is age was a factor or not in the timeline or water results to look over. Do you know what your water pressure was through the unit? If you do research a new purchase, I personally use BulkReefSuppy 6stage 150gph water saver unit with inline 3 stage TDS. My pressure is from 65psi to 75psi max, and never lower then 62psi. I have mine hooked into my laundry hookups. I believe anything below 60psi you need a booster pump for. If you have a significant other in this decision tell her or him there's also the addon of drinking water as well. That has worked for a few people
Sponge filter is too small for that size tank IMO. If it was a shrimp tank you could get away with it long term, but a 90g does have some volume, and plants need flow. A sunsun is a cheap canister filter solution to consider. Maybe 2x 304 models for the livestock you keep. The flow isn't great, but a lot of media room. If put together right, and not bumped into it should last awhile. Don't get caught up into the UV though in canister units, flow too fast to be effective, and they usually get so hot they melt/deform the media baskets. Replacement parts are stupid cheap via amazon. I just see this making life easier, and the tank less equipped in appearance. If you buy used a Eheim 2028 is one of my favorites, no longer made but see on CL time to time. Just a suggestion, usually FW guys switch to canisters eventually.
If you scape the tank right you can have a lot of flow, but areas where the fish can just chill as what you have does like slow moving water, (plants, rock) but the plants, and mere water volume in itself needs some flow. The chilli's will definitely do better in a smaller tank, but a 20g is big enough. I keep 35 chilli's in a fluval 7g ebi tank, and they thrive. They are dropping babies so moving them into a 12g long, but plenty of room, and that's why micro fish are so awesome. Big worlds in little tanks.
Glad the tank is looking better, I think you'll be just fine.