In my opinion you are gong to need the correct pipe to handle the sun and more than one role to be effective in lifting pond temp.
Very thin pipe matting in series is being used to heat up swimming pools effectively, although after 6 years mine started to perish from the sun, but it did lift my pool to 31 and 32 degrees after a warm day or two, it will slowly heat water over a period and will also cool down over a period, should not really spike temp to sudden, installed a bypass valve for when it gets to hot.
Albie what is the reason why you say that your water temp does not get more that 20 even in summer. With such cold temp, your pond is really at a disadvantage for growth.
I was concerned when I build my new pond facing south west but the summer temp went up to 28 and winter, well its 14 now, this is better than what I expected.