Is it to create convection currents to move water under surface? or what?

That is the theory but I have my doubts. The plants we grow have developed in locations without undergravel heaters and have developed to thrive in whatever conditions are naturally there without it. Most aquarium plants are bog plants and there is not a lot of water circulatin in the bottom of a bog and it also tends to be anaerobic. I think sometimes we think science is better than Mrs Nature. She is a clever lady and has been getting it right for a lot longer than we have. I have one tank heated with a heat pad and two without and all about the same size and I don't see any real difference.

i don't think any heaters are needed in this weather. I took my heater out of my marine tank and its still up at 28.5 degrees.... I have stuck a 3L bottle of water in the freezer and have put a big fan across the water as a temporary measure.... I think I might need to suspend my mh though.

One of the theories that I haven't seen discussed is that by providing undergravel heating you promote root growth and therefore higher nutrient uptake. In terrestrial plants if the leaves are at a comparatively higher temperature then you get reduced root growth compared to leaf growth. The actual temperature isn't the significant variable it's the difference between leaf and root.

But Alan points out that observations don't always match general theories. This is often due to the species involved, but is just as often due to reality not agreeing with the scientist in the lab :lol: .

Mother nature is a clever lady but she has only been working on aquariums for about a hundred years and hasn't had a lot of experience with undergravel filters, canisters and PMDD. So I'm not sure a tropical bog is a valid comparison with a temperate fish room :lol: .

I think Alan might agree with me that the main purpose of undergravel heating is to get us to spend money in the local LFS.

You may be right on all counts. My point was that these are bog plants that have evolved to suit that situation. These plants have been doing very well under the supervision of Mrs Nature rather than the relatively short time we have been trying to work out what is actually happening. If we realy new all the answers to what is happening in nature we would be God's mother in law.

What makes you think I'm not? :lol:

Do you have a daughter 6ft 4 with brown eyes and roman sandals? Can't be you then.