As for your 10g tank, 15w will probably sustain some lower light plants, assuming the bulb is suited for FW
plants. Do you know what color temp it is?
rule is great for medium sized tanks... but it can't really be applied to small tanks or large tanks.
You have 1.5WPG, yes... but it isn't comparable to having 1.5WPG over a midsized tank, like a 30 or 55 gal
. If you go smaller, it becomes even more clear what I am saying: If you have a 5 gal
tank, 1 WPG
won't grow anything. And for large tanks... say you have a 125 gal
tank... having high light on a tank like this does not mean you need 3 or 4 WPG
... That would cook the plants, not grow them
. I guess my point is, the WPG
"rule" is a great guideline for medium sized tanks. But it is only a guideline. There are a lot of factors that determine the quality and category of lighting you have... type of bulb, size of bulb, wattage, color temp, spread, PAR, distance, etc.
As for the LEDs... I will let someone else jump in on that. I haven't used LEDs over a planted tank yet. I know we have a couple of folks that frequent the forum who are using LED
fixtures and can give you the first hand experience on it, rather than what I have just read.