Years ago I was going to try building a cooling loop in my yard under the lawn to cool my big tank. I later figured out the tank is big enough and does not need any cooling especially since the metal halides were converted over to LED many years ago as well. Anyways I have a 3 ton rated titanium heat exchanger that is 1/2” OD tubing. If someone wants it please let me know. I paid like $400 for it years ago buts it’s been sitting around and I am tired of looking at it. It’s in a PVC shell but I suspect you could remove the shell as well if you wanted to use it in sump. I am just over in San Ramon.