I also have BBA for a long time, it proliferates since I increased the flow of the pump, I started the estimated index method, I reduced the light by 15%, do i need to lower more ?
As noted by the other posters BBA is 100% related to poor CO2.
CO2 is a very complicated subject and it is THE single most misunderstood factor in planted tanks.
CO2 is not just a button that we press. It is a technique that requires study and practice, like playing a musical instrument.
It is a sad revelation that over 90% of problems occurring in a CO2 injected tank are actually due to poor CO2. EI dosing has nothing to do with solving BBA. There are two main categories of algae: 1) blooms due to nutrient starvation and 2) blooms due to poor CO2. Both are exacerbated by having two much light.
For example, how did you determine that you have 30ppm CO2? Another sad fact is that no one actually knows how much CO2 is being presented to the plants. About 90% of the CO2 we inject goes straight out the top of the tank, so we have to make an effort to distribute the CO2 efficiently so that as much of the remaining CO2 actually reaches the plants.
We also have to reduce the demand for CO2 by reducing the light intensity. 60 watts of LED over a 15 gallon tank is far over the top and damages the plants because they cannot uptake enough CO2 and nutrients to use them properly. If possible the light should probably be reduced to about 10 watts.
We also have to clean the tank and mechanically remove as much of the algae in the tank because algae in the tank produce algal spores, which then bloom as more algae - in an endless loop. AS mentioned, frequent MASSIVE water changes help to rid the tank of bothalgae, spores and decayed organic matter.
The need for efficient flow and distribution of that flow has led to the general rule of thumb that the filter and pumps should generally have a flow rating of 10X the tank volume. That flow needs to be distributed evenly. I cannot see how the output of the filter/pump is oriented so I cannot suggest any improvements.
The timing of when the light is turned on versus when the gas is turned on. We usually suggest that you measure the pH of the tank water with no CO2, i.e., take a sample and let it sit for an hour or so, then measure the pH. Your target pH before lights on should be 1 full unit lower.
For BBA, one tool that we have now that we have not had in the past is Excel or any Glutaraldehyde equivalent. The tank can be dosed up to 3X the bottle suggested dosing as long as you do not have sensitive plants such as Vallis, Riccia or any type of liverwort or bladderwort. The BBA will turn pink/purple and can be removed from hardscape. Any leaves affected by algae must be trimmed immediately as they will never recover and will only drag the plant down even further. If you do not solve your CO2 deficiency, however, the BBA will return.
I do not know if the algae on the glass is diatomic. It appears to me to be more likely some form of Green Dust Algae (GDA) which is a definite sign that light is too high and CO2 too low.
The method by which you dissolve CO2 also has to be investigated. Is it in-line or is it a stone mounted inside the tank? If it is mounted in the tank a standard technique to try is to place the stone near the filter inlet so that the gas gets sucked up into the filter. This then enables the filter to chew up the bubbles and to dissolve the gas more. CO2 bubbles is NOT what you want to see. Bubbles rise an leave the tank even sooner. We want to dissolve the gas so that it is in an aqueous state. That keeps it in solution longer and will be more available to the leaves.
A lot of work to do on that tank. Roll up the sleeves and get the hands wet.
Cheers,