Selective Algae on Plants

Hi all, I have recently started a new planted aquarium, less than 2 months ago. I am not new to planted aquascapes but this is the first time I have tried it at such a small scale. 85L

I have had an issue with diatoms and green algae growing on some plants. I tried various techniques to get rid of it such as using R/O water, CO2 dosing, and reducing/increasing lighting and now have things dialed in to where I and the fish seem happy. The addition of some nerite snails, bamboo shrimp and 4 Otto's have reduced algae issues significantly. They have done an amazing job of cleaning it off the rocks and nearly all of the plants. What I cant figure out is there is one particular plant they simply will not do this on. I am not sure what plant it is but it is my only red one and am keen to save it as it adds some contrast to the green and grey in the tank but it is not doing well. There is some type of green algae growing on the leaves which I believe is blocking its ability to absorb the light it needs. Does anyone know why this might be? And why the clean up crew seem to avoid snacking on this? Please see pics I have attached.

Water quality wise ammonia is non existent, nitrates and nitrites are very low. Co2 is stable green on the indicator. Gh and Kh are in permissable ranges as I mix the RO with tap water to add some soluble minerals, obviously treating this with water conditioner. pH is 6.8
I also add some aquarium salt at the recommended quantities but do not have a way of measuring the salinity.

As you can see the green plants in the tank look healthy and are doing well

Easy to solve, but first, please always give us the numbers for parameters (these are GH, pH, temperature and KH) that you can test, and water conditions (ammonia, nitrite, nitrate). On the latter, what do you mean by nitrite and nitrate being very low, nitrite should/must be zero. And as said, what is the nitrate? The latter may be a factor in algae.

Salt should never ever be added to a freshwater fish aquarium except to treat a specific disease/problem. It does affect fish long-term. And it can harm plants too over time. It has benefit when nitrite is present, but not once that is resolved.

The fuzz on the leaves of the red stem plant (which is Alternanthera reineckii) is black brush algae. Otos or any other fish will not eat this (the two that might are not suited to this tank so won't get mentioned to avoid more trouble). It is caused by an imbalance in the light/nutrients, and it is corrected by restoring or establishing the balance. This plant requires stronger light and more nutrients than green plants, so it may be best to remove it and go with what does do well. There is also a nutrient issue affecting this plant and it is dying. Do you have data on the light, and are you using any liquid fertilizer?

Hi Byron Thanks for the response,
I'll try to give as much info as I can in future.
Ammonia = 0ppm
NO3 = 0ppm
NO2 = 1ppm
GH= 8°d
KH= 6°d
Fertilizer I dose with is tropica premium nutrition (see link) https://tropica.com/en/plant-care/liquid-fertilisers/premium-nutrition/
In regards to the salt it is something I have done for the past 10years. I have heard mixed advice on this in regards to stress relief and increases antibacterial resistance. I understand it's use as a short term treatment for various ailments and have read countless research studies on tolerance. Indeed salinity will fluctuate in their natural habitats between 0 and 1000ppm in the Amazon basin as well as the congo delta where most commercial breeds of tropical fish are sourced and their doesn't seem to be a clear definitive answer. In my personal experience (despite algae issues) I have never, touch wood, had issues with diseases or parasites or die offs in the fish I've kept and have always put it down to this. I am of course happy to take the advice of veteran aquarists on board

Sorry I forgot to mention lighting! These are manufacture installed LED Blue and White light system installed on the Superfish 85 aquariums. From what I understand the blue light spectrum is more critical. I have both the blue lights and white lights turned on for around 9hrs a day. I find anymore than this increased algae issues. I know some people run for 12-14 hours and even on my previous 300l planted set up, I could only achieve these light lengths after about 8months once the plant growth was excessive enough to starve the algae of nutrients.

Did you mix nitrite and nitrate...NO2 is nitrite and you have it at 1 ppm. NO3 is nitrate and you have it at zero. The fish do not seem too bothered so presumably the N)2 is actually zero (?).

The Tropica fertilizer should be fine, others use it with success, and the nutrients are there. However, as they say themselves, this is a supplement for low-tech or natural planted tanks and you seem to be using CO2 which takes you to the high-tech method. But, if the other nutrients are not available, the CO2 will likely promote algae. Everything has to be in balance. And the red stem plant needs good light and nutrition.

Balance brings me to the light. A mix of red, blue and green, which has a Kelvin in the 5000K to 6500K range, is ideal as it is close to the sun. Green plants appear green because they reflect green light. Same holds for red plants, so they need even more red. Red and blue drive photosynthesis, primarily red, so it is important as a major wavelength. And if it is being reflected by red-leaf plants, there has to be even more of it. The blue diodes are useless here, the white are the principal lights but this depends upon the make-up; if it is high red/blue/green in the Kelvin range, it is good...but red plants may find it weak. And thus algae has the advantage. I gave up red plants some years ago for this reason, except for a few crypts that were brownish-red, and my Red Tiger Lotus, but the latter produced floating leaves (what I wanted) that were burgundy and being floating closest to the light.

As for duration, you could reduce it to 8 hours and see. I got my tanks down to 7 hours, and the black brush algae never increased again for several years following. The increased intensity and duration of daylight in summer can cause this algae because it is enough to off-set the balance. I covered the fish room windows completely, and that solve this.

Salt is detrimental to freshwater fish, especially soft water species. It has absolutely no antibacterial benefit, and it does not promote stress relief, guaranteed. An old wives tale. Like the earth is flat, yea sure. I have an article proofed by Dr. Neale Monks that explains the detriment to fish.