The diatom/brown algae is harmless itself but it is triggered by high organics levels and mini ammonia spikes, as in the range of 0-0.25ppm. I did that recently in my hillstream loach tank when I put repashy food that ended up blown in the water column and then perhaps settled in the substrate and wasn't dealt with by the tank fast enough, despite my big water changes, so it ended up polluting the tank. However, I kept doing large water changes and the issue is now down to a minimum. The issue with diatoms is that you need one time trigger and then it lasts for weeks...
High light levels also promote its growth so I'd start from there as a control measure but it is not the trigger. My hillstream tank is subjected to extra light as part of the upkeep. Also, you've got gravel that is possibly now full of organics over the years of build up and no plants to utilize them. If you're going to eventually move the fish, I'd change the substrate altogether to sand
And a bigger tank will help long term Also, I'd not sacrifice nutrition for water quality and vice versa. A feeding once daily is enough for my own clown loaches to look good and be healthy though I often give a smaller feeding later on, and roughly a large weekly water change is also enough to maintain water quality, but then again I have a much larger tank and very low stock in comparison, plus plants that mop up nitrogen compounds.
Apart from that, since you're going to move eventually and move the fish to a new tank, I'd just ignore and keep doing as many water changes as you have the time for/possibility, and ignore the brown algae.
Diatoms are an important part of the food chain of fish and they actually help maintain the water quality, same as plants would. If you've go excess of something, there's an organism popping up to utilize it.....It is just a sign of something going on.....The only real long term solution I'd have is more water changes, if the fish remain in the same conditions, and of course clean filters, and clean substrate...though it's hard to do that with gravel as it traps a lot... and siphoning it out just releases nasty stuff in the water column, invisible to the naked eye...which can trigger diatoms as they're normally an organism living in the substrate in anoxic conditions...
I don't known if that made any sense at all.
but if I am to buy a new tank, it will be acrylic for sure. I've had enough of glass tanks. They're also a pain to move, even when they're broken and one wants them out the door....