I have US fissiden in both my 1 feet and 2 feet tank and they are in T5 and LED light environment. I don't see any different between the two, but you may want to measure what is your water temperature. That maybe a reason why it is turning brown. US fissiden is a quite easy to grow and not so demanding plant but the same can't be apply on its cousin, mini fissiden, on my encounter with it.
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