The immersion coil should work for what you need it to do. If not, just another in the loop. You can get a brazed plate hx made of stainless steel, that would do the job better for less space and more cash. Just remember the BPHX will need a filter so the fish poop and food won't clog up the passages. The open coil will not.
I finally got to the grow store. I bought a quart of custom bloom and 8 oz of pH down. The Ph down is weaker than the Hcl I started out with, but is phosphoric and nitric acid blend. Both of these acids are plant food. Guaranteed NPK analysis is 1-5-0. For these 2 systems for the year, i might not even use the whole 8 oz if i switch to rain water.
Before pics from Monday:
During pics:
After pics today:
I set the pH on the solution before it went in the containers. It looks like the flower box mix i used has a lot of buffer in it. The petunias will green right up once I get the pH to settle around 6.0-6.3, it takes a couple shots of acid a few days apart to balance. I'm better off not adding too much at once, learned that one the hard way before.