It will be interesting to see how it all works in the end.
ZombieEngineers remark is technically correct. However, the second tank (without coil) is so small that adding a pump there will be extremely impractical, effectively impossible. Therefore I choose the Eheim 2400, which can be positioned outside the water. It is outside the second tank. I could put it in air, but I just placed it in the big tank because of space convenience