The other issue I have is that I'd really like to increase the light
levels in there. I presently have 15W, but I feel that's not enough
for the plants ....
.....Bearing in mind the
depth of water is only 12", should I bother with more than 15W?

For a 15 gallon tank, in order to grow plants well, I'd go with
atleast 30 Watts. Preferably 45 Watts. Also look into DIY CO2.