Having lived in England a few times in my life, and having visited in various European countries, I suggest that the reason undergravel heating coils are recommended is because most homes are not centrally heated to the extent that homes are in the US or Canada. Electricity is much more expensive and people wear more clothing indoors than we do here. It's quite possible that in these circumstances, regardless of the presence of current-making powerheads etc. the temperature of the substrate would be significantly lower than the water. And if there are fry in the tank, powerful currents are a definite no-no. I have noticed that the substrate in one of my tanks that is close to a window that is not properly installed and hence leaks cold air in the winter will have a noticably lower temperature than the water. Installing styrofoam insulation underneath and behind the tank helps somewhat provided that a small powerhead is used in the tank. This is not a breeder tank, so I don't have major concerns. However, I can say that most plants do not seem to appreciate the temperature gradient. Their growth is slower that when the entire tank has the same temperature. G. Kadar ------------------------------------------------------------------------- This is the apistogramma mailing list, apisto@listbox.com. For instructions on how to subscribe or unsubscribe or get help, email apisto-request@listbox.com. Search http://altavista.digital.com for "Apistogramma Mailing List Archives"!