When I setup my 75 gal tank 3 years ago, I did not put enough amount, which I learned later from the various sources, at least 3 inches of depth needed. I planted water sprite and wisteria but they did not grow tall as I expected although I use 50% Flourite (mixed with other substrate). The height of tank is 18" but the plants I have now never grow more than 10 inches. I use Whisper 5 outside filter, no CO2, water change 20-30% per week. I keep many guppies and a few cori cats. My question here is whether or not if it will be possible adding more substrate to make up the required depth, which may affect the growth of the plants. I am fairely beginner for the planted tank and all other people here are very experienced so I wanted to obtain some advises. I have been tempted to redo my tank from scratch (I don't know what I would do with my lovely guppies though). My intention sounds really stupid but if somebody can give me a good advise, I will appreciate your opinion or instruction. Thanks.