Re lily depth, have a look in "rock and water garden expert" by Hessayon, you may find it in B&Q or garden centres. If you want to get really posh "complete guide to water garden plants" by Helen Nash and Steve Stroupe, ISBN 1-4027-0954-4.
Most UK lily labels carry a depth indication and also have a look in B&Q at their dry packed lilies.
http://www.victoria-adventure.org/ is a good site for lilies although I dont know if they list depth.

Re fertiliser tabs/pellets, some if too close to the roots can burn the roots, for this reason I have started mixing loose slow release osmicote? beads in the soil where possible rather than add a pellet in one location. It will be interesting to see the results, from memory these are a Miracle grow product but there are probably many makers.