How to keep my poor hoop house barely warm

I built a hoop house over my pond to try to keep it slightly above freezing. The problem is when I built the pond I had not considered using it  year round so where it is doesn't get much sunlight. I used the PVC poles and 6 mil plastic but it's not enough and only makes a few degrees difference even during the day. Question I have is what can I do to improve performance. I would be fine adding another layer outside or inside but with the very limited sunlight I wonder if it will make any difference or if I should try something different?

Hi John! Welcome to permies!

Would you be able to post a few pictures of your situation?
I'm having a little trouble understanding what's going on.

John Jay wrote:I built a hoop house over my pond to try to keep it slightly above freezing. The problem is when I built the pond I had not considered using it  year round so where it is doesn't get much sunlight. I used the PVC poles and 6 mil plastic but it's not enough and only makes a few degrees difference even during the day. Question I have is what can I do to improve performance. I would be fine adding another layer outside or inside but with the very limited sunlight I wonder if it will make any difference or if I should try something different?

Why do you want to keep it above freezing? That might help folks come up with ideas.

Each layer of hoop adds at most 10F. so if you had 4hoop house nested you would have up to 40F temp for the air. Not too sure how much the water temp would increase by.
What is you avg temp by month (max and min too(

Do you have a way to keep the water moving?

I would think the most important bit of info is how far north/south are you? how much energy do you get from the sun? Right now I get 0.3kwh per m2 (averaged over the day) For me at this time of year a greenhouse/pollytunnel adds no heat at night, it only protects from wind/rain.
A single skin pollytunnel will hold around 1-2C higher than the outside air at night that is it

The hoop house is over my pond and I want to keep the pond for completely freezing over so that gases exchange during winter.  I am in CT in USA

Is there any way the access to sunlight in this area could be increased?

Are there rocks you could use to increase the thermal mass of the pond?

Where is the pond in the context of your property?

How deep is the pond? Are you trying to protect fish or plant life?

It looks like the pond is about the size of a bathtub.
We are still waiting for your winter temp/hardiness zone but for now I will assume a median of zone 6
Given that it is not below the frostline, and no annualized thermal storage and only 1 layer of plastic as insulation, or a bubbler to mix the water to prevent stratification. You have alot of challenges ahead.  

Howdy and welcome to Permies
Keeping air in the water is important. A couple options I have experience with in this situation.
They make floating pond heaters, but they use a lot of power, same with aquarium heaters. The latter I tried, as I wasn't prepared for Winter when I setup my Koi pond three years ago which is made from a hot tub.
First thing is to add a inexpensive ($10) aquarium air bubbler  and plug it in. Put the bubbler in a shallow part of the tank so the hibernating fish in the bottom aren't disturbed by the bubbles. The bubbles will keep the water from sheeting over in ice.
I went further.
I asked around and found a neighbor had some bad hay in the field I could have for free. I surrounded the spa with the bales going a few bales high then laying a sliding glass door over the top. that sealed it.
It gets down below zero here in northern New Mexico several times. I had hoped the hay would rot in place and make some heat, but I have since learned more about how to make biomass decompose since I've been actively learning here at Permies.
The hay did insulate the pond from sub-freezing temperatures.
You can look at what I did after learning about hugelkulture for this year Garden-Hugelkultur-Koi-pond-fun
Good luck
Brian

I don't like being the nay-sayer, but I don't think what you are trying to do is possible without an external heat source (other than the sun).

John Jay wrote:The hoop house is over my pond and I want to keep the pond for completely freezing over so that gases exchange during winter.  I am in CT in USA

I assume this is for fish? They make special bubblers/de-icers just to keep some oxygen flowing for pond fish.They don't appear to be terribly expensive, $100-$150 for a combo unit that works for a 2000 sq ft pond. Here are a few: https://www.thepondguy.com/product/airmax-aeration-and-thermo-pond-de-icer-combo

The bubblers, de-icers and combos are not designed to warm the water or keep the entire pond surface ice free. They’re meant to keep an opening for gas exchange, which is all that’s needed for the fish in your pond.

If your vent does freeze over during extra cold temperatures, simply pour hot water on the ice where the hole once was. It’ll melt the hole back open – and your fish can breathe a sigh of relief!

https://blog.thepondguy.com/tag/gas-exchange/

Ok I have something to try with the bubbler. I ordered 300 ft of airline tubing. Once it gets here, I'll cut off 120 ft for each bubbler and run it out to the pond (it was much cheaper to run airline out to the pond than an electrical wire). Hopefully I don't loose too much air pressure en route... I suppose if it is really airtight maybe it will still the pressure. I have a hard time imagining how bubbles would keep a pond from freezing in a spot but that seems to be the first line of defense everyone recommends so I'll start there and if it works next year I'll buy some kind of high end tubing and bury it in PVC.

Check craigslist for older water solar heaters.  I picked up two panels and a tank for $300. Run a pump to push the water through the panels and back to the pond.

If your winter temp gets down to -30 and the soil freezes to 10ft, no amount of bubbler/straw will help.
But if your soil only freeze to 1ft, Then there is hope.

So the species I am going to keep in there are newts. They need a long cold winter to breed properly so I didn't want to heat the pond plus I don't want to spend a lot of money on electricity. I also was reluctant to run electricity to the pond since I am not really knowledgable about electrical wiring. I pond a pond aireator off amazon and 300ft spool of chinese air tubing off ebay. I hooked up the aireator and run 100ft of the tubing out to the pond and much to my surprise here it is a cold snowy night and it has a big hole made in the shallow section of the pond. The real test will be how it does during the coldest days of Jan and Feb, of course, but I am thus far amazed that this concept is actually working. The aireator has a place for a second line too so if the tube ever does condensate and freeze, I can run an emergency line with my extra tubing. If this does work, I'll trench em in some condiut for next year when there are actually animals in the pond.

So you have a pond in a greenhouse to breed newts?  That's pretty cool.  Are they for pets, meat or eggs?

For pets. A lot of salamanders are fairly toxic to humans if you eat them.