I'm no expert(yet). But I have had similar problems before. I have a 55g with Live rock/ live sand, couple o' clowns, blenny, crabs, snails, sea star, and a bubble tip anemone.
What other inhabitants are in the tank and for how long?
What kind of filtration?
The reason I ask is that three months may or may not be enough time to have a good biological filtration base. Depending on the biologic load of the tank, you may be in the middle of the cycle.
That is why you should probably check more than nitrates. Check nitrites and ammonia as well, and keep a log, especially when running in a filter. Anemones do not do well in poor water quality, and this means more than just nitrates.
Another thing to be concerned with for anemones is that they do need sufficient lighting. Many people will recommend that you have at least Power Compact lighting. I have kept some basic anemones for many years with regular flourescents. However, the bulbs lose their strength over time, even if they still seem to work. I would replace at least one of those two year old bulbs.
Good luck with it!