Having various brands of tankless, if temp is raised to a certain point, you start getting a build up of scale inside tankless w/h,thus ruining tankless. If me, leave tankless at 120 degrees, and do like you said use dc pump and tweek pump flow. Problem with aquarium dc pump, may not last long, heat my effect it. May need to use regular recirculating pump and control discharge with ball valve. And once a year need to descale tankless, every tankless has its kit to descale as maintenance protocol.My plan is too use a titanium heat exchanger. I posted a picture of my plan in an earlier post. I'm going to use a DC pump to control the flow through the heat exchanger. By changing the flow, I can control the amount of heat transfer to the aquarium water. I'm sure it will take some testing, but I'll get it figured out. I also have my plumber looking into a separate smaller tankless unit just for the aquarium. I can then play with the water temp of the unit to see what temp works the best.