Re: peat as substrate

Dont add composted manure! The organic matter is way too high ... 
generally a soil substrate should not have more than 20% organic matter 
or the macrophyte growth decreases. Peat is not a problem, however, since 
the organic matter is 'refractive', that is , it is not easily oxidized. 
One of the main benefits of a soil substrate is that it is anaerobic ... 
this solubilizes iron, phosphorus and converts N-NO3 to N-NH4 which is 
preferred by most aquatic plants. You do not want these minerals leaching 
back into the water, in fact a heating coil works the other way by 
bringing water into the soil (or so I'ver read).
The best substrate for growing aquatic plants is a silt or sandy loam 
with about 10 to 20% organic matter. The trick is to find some that is not 
contaminated with chemicals.

Dr. Dave