When I first started this tank, it was a 10 gallon. The only thing I have left of it is the 15 watt light. Since the light was designed for a 10 gallon, and is now on a 15 tall, is it enough for what I have in it? Or should I have more. If I need more, since I cannot find a more powerful 18 inch bulb, should I put another fixture, bringing the total to 30 watts? Or am I ok where I am?