Get some K2SO4, dose that by weight/volume like you might if you used KNO3.
I've run NO3 up pretty high for several weeks to see if there was any adverse effects on a well run aquarium, never was able to induce any negative effects after 8 weeks at 80ppm.
10ppm or more certainly is non limiting for any and every alga I know of.
Plants might do a bit better at say 20-30ppm than some species.
So anything higher is really just excess, but obviously cannot be feeding algae nor harming plant growth either. Hydroponic solutions run around 200-300ppm of NO3 and what are often used for growing aquatic plants.
Hard to see where folks get the rational for limiting algae, or stunting plant growth by excess ppm's.
I guess they just make it up and see if they can find some more "me too!" folks to join in with them.
Anyway, I'd refocus on the CO2, and stability factor.
Light seems good, 1.5w/gal of T8 lighting well spread over the aquarium sounds pretty good to me.
It's not pushing growth or demand much.
So getting good filtration, good CO2 and good circulation should not be that difficult.
I've wondered if repeated prunings, uprooting might be some of the issue for some folks. I did this a lot, and rarely ever just topped plants and left the bottom remaining.
Now I tend to a mixture of both topping and uprooting.
Perhaps less uprooting will improve things also.
You can always look at the aquarium with respect to CO2 this way:
do a large water changes in the morning, then look at how the aquarium is doing later that night after 6-8 hours of light.
Is the aquarium doing this on the non water change days?
Why not?
Same nutrients, same light for the most part right?
What else is different?
1. Perhaps you clean the filter, wipe the glass, clean that day.
2. A big issue I believe, is that the plants' boundary layers are disrupted and then cleaned by the current and wave action with a large water change. Some leaves are exposed to the air.
3. CO2 rich tap water, other dissolved gases bubble and break up the boundary layers, and add more CO2. The sticky gas bubbles attach and pull off algae on leaves.
If you do a large water change each day, for say a week, or do them say 2-3x a week, the aquarium should really look good, clean and the plants should grow in very nicely.
Seems excessive and it is. However, it's done more as a reference as to what could be.
You do this to see where the CO2/current, filtration, CO2 delivery system could be improved somehow, without harm to fish.
It's not a simple thing.
The water changes also help to get rid of some nagging problems, or keep them at bay until you get around to addressing the root issue. Same with limiting nutrients, or doing a blackout 3 days on/ 3 days off etc.
Adding Excel or Easy Carbo does a similar thing.
Since more water changes requires work/more effort, it's a temporary management tool. Once the aquarium starts to look good and seems to be on a good path to recovery, you can reduce the work.
Once out of balance, we have to work 2-5x as hard to get it back to the nice state we find that looks nice and is easy to care for and garden.
My own dutch style tank is getting there very quickly.
Same management things I suggest here, fixed that tank.
I had GDA, GSA, Green water and Spirogyra recently.
Sorry, no magic cures.
I wondered a lot about what was causing the issue with that tank vs the others.
If I did not have all the past experience, and reference tanks to compare to, I'd be very interested in seeing about less nutrients. I'd be very tempted to suggest that route.
Fortunately, in my past, I tested nutrients very well and effectively on a reference system and I have several tanks running to compare to. It all you have are assumptions, one aquarium, no way to obtain a reference aquarium(which is why you have issues and where you want to get to and be able to have enough control to make such a stable algae free aquarium), I think it's very hard to know what to believe.
I went through that phase myself.
All I can say is to approach it like a research test, make everything else independent, then test on a referenced tank if you want to see/test.
If all you want to do is to have a nice planted tank, then focus strongly on CO2 and use low light (you have that), a good sediment with some nutrients, and dose the water column.
Good filtration, good current and good gardening.
That's most of it right there.
Regards,
Tom Barr