Please test all your parameters and post them (ph, KH, NO2, NO3 Ammonia)
There's several factors that enhance brown algae growth which need to be either eliminated or excluded from the 'cause list' step by step for your tank.
One cause could be higher nitrates.
Another cause can be low light. Now this does not simply mean to dial up the lighting durance, but possibly exchange for better bulbs - So what type of bulbs do you have (full spectrum?).
It often occurs in new tanks due to lack of oxygen and abundance of nutrients.
So in ratio to your tank size, how many % would you say are covered in plants if any? Or maybe you can take a picture of the tank?
Brown algae or any other type for this matter is manly caused by an ecosystem within he tank that's out of level, meaning something is too high and other levels are too low. To permanently eliminate your issue, the cause must be found and eliminated.
Short term solution: If you have areas like the glass or decor covered: Take decor out and brush it clean. Use a sponge to clean the inside of your glass (careful: don't use any brush or sponge that previously was used with any cleaners or chemicals).
Hate to tell you, but to solve it we'll have to go through several different matters in your tank and eliminate 1 by 1 and this may take time and patience.
On a side note, yes the kids fit your tank right now, but as adults depending on the exact species you're housing you're looking at a size of anywhere form 2-4" per fish and their active behavior, that's why once they grow, they'll simply outgrow the 29g. But there's always the option of upgrading to a 55g and use your 29g for another smaller species of fish