I've experimented with this algae problem a bit, and I don't think it is light related. The diatoms have been very bad in all three of my aquariums. In my 5 and 10 gallon, I experimented with light (because its cheaper to do that with smaller tanks compared to a 75 gallon). I've done from 2 watts per gallon to 5 watts per gallon seeing how it would effect the diatoms - and the higher the light, the worse the diatoms get. They were much much worse in the 2 watt per gallon than in the 75 gallon that has 1/2 watt per gallon. In the 5 watts per gallon, they were incredibly bad. I'd hoped to change them to green algae by the higher lighting (because I think green algae can be kind've pretty), but it just remained diatoms and become more prevalent. Also, in the wall panel of the 75 gallon that is very close to the 10 gallon (which has brighter light) I have more diatoms than anywhere else in the 75 gallon.
I also experimented a lot with plants in the 10 gallon before making any headway at all with the diatom problem. I used Wisteria, which as I understand consumes nutrients pretty well, and it didn't make much difference by itself. The diatoms were growing on the leaves, I had to keep cleaning them off. Eventually I got a light-fertilizer-plant amount balance that keeps the algae in there tolerable, but not ideal. I also have some Cherry Shrimp in there as well, which will nibble on algae, but don't make much difference at all.
The thing with the 75 gallon and diluting is, that's a pretty big volume of water, and it would be expensive to either buy RO water to dilute it or get an RO unit, as I do 40% water changes a week and its a big tank to begin with. I pay for the things my fish need healthwise and to take care of them, but something that just looks bad in the aquarium to me I would rather take care of by cheaper means, if possible, I don't have an endless supply of money. And I wouldn't mind having an excuse to get a new fish, either.
As for the chemical, Proper pH 7.0, it does not actually harm plants. The reason the instructions give for not using it with plants is that it is a "phosphate buffer" and plants will consume some of the phosphate and cause the pH to swing. It has occurred to me that the phosphate from this pH adjustor is causing my algal bloom. However, I only use it in 1 of my 3 tanks and all 3 tanks had the problem at the same level, and I was using it for months without it causing this issue (the months I had the tank set up before the water changed). Also, my boyfriend who lives just over the state border (different water company) also uses this chemical in his tanks and has never had diatoms in his aquariums. I know a lot of people don't like adjusting the pH with chemicals, but my pH is very high out of the tap (8.2-8.4), and I have been using this chemical for 2 years during which the pH has never swung and it has kept it in the ideal levels for my fish. I've tested it many times during different days of the week and different months of the year to make sure it stays the same, and it always has. I've researched it in regard to Cories as well and have found no reports of it causing any issues with them, and I've never observed any abnormalities in my Cory fry from babyhood to full growth in this tank. My mother used this in a guppy breeding tank for about 10 years too, and there were never any abnormalities in those fry either, so I believe it is safe.
I don't know why the diatoms don't cause it to swing, though, if they consume phosphates too...but for some reason they don't. Never thought about that. Well, its good they don't. Nevertheless, I would not like to push my luck with adding plants when it says in the instructions not to. And I don't really want to change my method of acidifying the water either, because using this chemical has been more precise for me than using peat and bogwood, which I've used in the other aquariums so that I could have plants in them.
I guess I just assumed the Ammonia in the tap was causing it because I'd never had any diatom issues before until the tap water company started adding chloramine in. I was experiencing issues with the algae and elevated Ammonia levels. I was losing some Panda Cories because of the Ammonia levels. I was very confused because I had made no changes to anything. I'd called the tap water company and asked for changes in the water, and the chloramine and Ammonia were the only ones they mentioned. However, I had to call a few times before I got answers on it, and I wouldn't be surprised if there is something else like Silica that they changed at the same time and just forgot to mention. I was sooo mad they didn't notify me ahead of time about that Ammonia, so I have a tendency to vilify it - if I hadn't tested the tap water and started using Prime I'm sure all of my Pandas would've died, I'd lost a few before I found out the problem.
I have an adorable Olive Nerite Snail named Callahan in my 5 gallon, and he has taken care of the algae completely in that tank. There is no sign of it anywhere. (And, he produces enough poop that I don't need to fertilize the plants in there! He's a double-worker
). I've tried light changes, I've tried plant changes, and for a while I tried DO water in my 10 gallon (that did help, but again, it would be expensive in the 75) but none of those have worked as well as my little algae eater. And I thought it would be worth a try in the 75 gallon to find something that was compatible with everyone. I'd get another snail, but the water in there is, in my opinion, too acidic for a snail. I think he would have shell erosion, and I don't want that to happen. I also have misgivings about trusting my Praecox Rainbows with something like Amano Shrimp. And I know Otos are safe with fry, but I've had trouble getting them to survive in my tank - they don't acclimate into it well, I lost a whole school of them I added over a week or so and I don't want to try again if it means they may just die, I don't want to risk their lives. I understand that getting a fish to fix your problem isn't ideal, but I'm at the end of my rope with this algae, its just so darn stubborn. I spent an hour cleaning my 75 gallon decorations and tank glass of it today, I do that almost every week, it just keeps coming back and there's so much of it. I've looked into algae eaters a lot too, but most articles on it don't mention if they are safe with little fry. The best I've been able to find is a list of fish they are compatible with. I'm also having trouble finding this info on a forum search. That's why I've asked.