My lights work with a timer.
Light on at 9:30am and light off at 10:30pm every day.
I have tested my tap water and it does not have nitrates at all.
I do a partial water change once a week and I change 50% of the water.
I always clean the substrate as much as I can and do a maintenance to my filter once a month with water from the aquarium.
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OK, down to some specifics. Reduce the tank light duration, this is clearly one major factor. My tank lighting is on for 8 hours daily (also a timer) as i worked back from the original 12 until BBA ceased to be a nuisance and this was where the light duration ended up. You can go down more if needed, with six hours usually the minimum suggested. But for now, I would reduce it at either end (or both) depending when you are normally home to view the tank; no benefit in having the tank in darkness when you are there each day to enjoy it.
I would be aggressive and go right down to 8 hours. Plants and fish also need sufficient total darkness (meaning, no ambient room light, but complete black darkness) for several continuous hours each 24-hour period, so work around that too. In my fish room the tank lighting comes on at 9:30 am and goes off at 5:30 pm, which allows me to use daylight for the "dawn" and "dusk" periods. Light significantly impacts fish so the tank lighting should not be suddenly on or suddenly off, but with sufficient ambient room lighting before/after.
To the nitrates. Dealing with high nitrate occurring within the tank is much easier than if you had these in the tap water. Increase the volume of the water change, I do 60-75% each week. Keep the filter well rinsed. Don't overfeed. I didn't ask about the fish load, but if it is in balance for the tank volume that should not be an issue.
It will take a couple weeks, and what you are wanting to see is no increase in the BBA. What is there will remain, or you can pull some of it out, up to you, but we need to stop "feeding" it with excessive light and/or nutrients to get it under control. And not increasing is what you want to see to show this is achieved.
You can clean the filter under the tap. I have been doing this for 30 years. Once the tank is established, this is of no consequence. Your plants are taking up most of the ammonia anyway, not bacteria, plus the greater colony of bacteria is in the substrate not the filter, and cleaning the substrate with the water change vacuuming is not going to dislodge it at all. Keep the filter well cleaned. All that brown gunk as I said is organics and that you want to get rid of as much as possible.
Having white gravel for the substrate is ironically also working against you, as this reflect light so the light down there is even more, feeding the algae.
