How small is small? Our 100'x120' pond freezes almost every winter, despite a constant spring running into it from a pipe. A small area right at the inlet doesn't freeze, but it's not enough open water for the ducks to swim. I have to break ice most mornings and some evenings, and haul the chunks out of the water. My least favorite farm chore.

Yes we do have a very different view of the zones. Mainly for me, that is because I think the USDA zone map is "somewhat" useful in a limited way if I apply it to plants that I want to landscape my yard with. Its a "loose guide" for me, no more, when landscaping. Mainly because it only deals with COLD temps. You have to use a separate criteria for HEAT here in the South. There's no one map. I think that's a shortcoming. I know a separate heat index map does exist. But as far as the USDA zone map is concerned, I find it a lot more useful for my greenhouse. I am in zone 8B. However, I am still able to grow many plants in my yard that are supposedly only labeled for zone 9 and above. If I gardened by what books said about the zones certain plants would live in, and paid attention to the zone maps, I would be shortchanging myself on growing a lot of neato plants that actually do very well here. I do find the concept of "zone" imminently useful inside the greenhouse though. I have plants growing and blooming in a zone 11 environment that are supposedly "zone 10 and above" plants that friends of mine in South FL zone 10 have not been able to get to bloom in their yards. I have to wonder, if I turned down the heat, would they still do as well? Probably not. When I first moved to FL in the very early 1990's from Los Angeles, I was stunned at all that could be bloomed here, in the ground and even just in CONTAINERS in my zone 8B, that could not be bloomed in the zone 10-11 So Cal environment. Certain gingers, heliconias, vines, orchids, they will do so well here even if you have to stash them in a freeze, and they wouldn't do well in So Cal even in a "frost free" environment. I asked "Why?" many years later on a forum that was heavily weighted with West COast people, and the answer seems to be "heat accumulation". Even though So Cal where I lived was a zone 10-11 and rarely if ever actually freezes, its too cold at night most of the year, and too cool in the day most of the year, to afford the continuous heat needed by certain plants to thrive and bloom. So when I set up my greenhouse, I had to think, "How do I want to do this?" Because actually, In FL, its much harder to cool the greenhouse in summer than it is to heat it in winter. I decided that I wanted a "conservatory style" planted greenhouse instead of a greenhouse that had benches to set stuff on, and that if I was going to go to that type of setting, I wanted it to be a jungle with everything that I could never leave out all year here. So if I was going to commit to THAT, I decided, zone ELEVEN conditions were what I wanted. Anything that can take a zone 8B winter, even if it goes dormant and comes back as a "perennial", or gets leaf burn but survives, does not merit the zone 11 greenhouse. Only stuff that would never bloom or live in zone 8B, or is expensive of somewhat rare, gets to be in the zone 11 greenhouse. Maybe that's a simplistic attitude on my part. I don't pay a lot of attention to how much it costs to run the greenhouse. I only keep records for tax purposes, because I do run a small mail order plant business and have to report income and expenses. If I was a huge business entity I would probably think I needed to have a more formulaic approach to the greenhouse, but I'm just not. I admire people like you who have all that scientific stuff going for them. Its just not my focus. I'm more into just cranking up some Blue Eyed Christ, getting out the fertilizer and piddling around in my greenhouse. I don't like to think about it too much.

How much heat you need depends on your location. You've already beefed up the important part: the insulation. I'm assuming that you put the lions share of the insulation overhead, as that is where it does the most good. You might also install those pink foam sheets internally against the walls and ceiling. They won't get wet and soggy from condensation like fiberglass or cellulose and will provide better R value. The hottest bulbs that you can burn are halogen. So look for some full voltage halogen lights that you can put in there if you are in a really cold climate. Otherwise plain old incandescent will do, if you can still find them.

I guess that if your pond depth is really 3.5' that your volume may be correct. In any case, 900gph is way low for the generally suggested ratio of flow to pond volume. BTW, I have an auxillary pond that has no flow. It can be a total mess of muddy water after a big rush of accumulation from our French Drain. However, this crud will precipitate and this catch basin will become completely clear, again with no pump! I have been very lucky and never had pea soup type issues. I am assuming that this is what you are referring to and not just some string algae, which is a good thing anyway. You failed to indicate your location or garden zone. It may be useful information. I hear that ponds without much plant cover are more likely to get algae problems. I suppose hotter brighter conditions increase the problem. My aux. pond (without any flow) is in the woods with minimal sunlight. There are plenty of in pond pumps that work fine. I don't actually understand where your crud that restricts flow is collecting. I have a 2500gph pump contained inside its own overall slitted case. I remove leaves and other crud that get sucked to the surface every few months. Perhaps your pump is so close to the surface that it catches most of your leaves? Perhaps at the bottom instead, right where leaves fall? BTW, the bottom is not the right place for a pump because a big plumbing leak/break will drain the pond.