The correct answer to this question is indeed "Only one".

Biofilters in a Recirculating Aquaculture System (RAS) primarily convert harmful ammonia from fish waste into less toxic nitrate (nitrogen cycle), making Statement 2 correct. This is done through a process called nitrification, which involves beneficial bacteria in the biofilter.

Statement 1 is not accurate because the main purpose of biofilters is not to remove uneaten fish food, but to convert harmful elements in fish waste into less harmful substances.

Statement 3 is not true either. Biofilters are not generally designed to add or increase nutrients in water such as phosphorus.

So overall, only one out of the three statements listed above is proper in the context of biofilters in RAS.