Does anybody remember the name of a book that involved an alien race that used some type of power system that used iron rather then uranium to produce power?  Started off with the alien force collecting iron from human ships and from the hemoglobin in blood. The alien force returned to their planet in time to transport some of the iron to a city on their planet that was under attack from some “fishes of the greater depths” of the planet’s ocean.

I found the series.

https://en.m.wikipedia.org/wiki/Triplanetary_(novel)