FlytrapCare Carnivorous Plant Forums

I was wondering recently, I saw my plant has gone into some kind of slow decline, which in this country should never occur due to constant temperatures, but I wonder, is it a lack of nutrients or something else, I have it in a 4 gallon tank with a layer of natural sand from a river beneath, and it is the only plant inside, no algae nor anything else, what I do is only refill it with rainwater when the water goes down, but the plant has not grown in some time, I had even made cuttings some time ago, which I gave away because the plant was growing really nicely.
Is fertilizer needed in a small dose to aquatic Utrics?