Tank Turning Brown

my tank plants and decor are going brown .I have been cleening these every week but they quickly become brown again. I think it could be Diatoms .Have increased lights on time from six to eleven ,and it seems worse .I have some live plants and belive lots of light is necessary.Any help on getting rid of this please?

I had the same problem when I set up my tank, plants covered in brown algae, I bought 4 otocinclus, one week later all the brown was gone and it never
came back.

Thanks all. Doing regular weekly water changes, cleaning and lights ok, reduced lights on time back to six hours. I have a few live plants, and yes using fertiliser, plants are vallisnaria and anubias. Tank has 12 tiger barbs in. Are ottos OK with these?

Others eg @Matt are much better at explaining plants' requirements than I am. However, 11 hours' "lights on" seems very excessive to me - 8 hours is usually much more the norm/upper limit. Do you use any plant fertiliser or other products?

Just catching up on this and tbh I've not got a huge amount to add, and agree with everything said so far.

With just vallis and anubias I would probably reduce the fert levels to half the recommended dosage. Vallis tend to take its nutrients from the roots and anubias is very slow growing.  Advise you don't dose one week then go for half levels from then onwards.  You would be better with root tabs possibly for the vallis and not bothering with the ferts apart from micronutrients at a low level.