We have been enjoying our stock tank for six months. All of the sudden we have a rash of rust colored spots all over the inside of the pool. See attached. We have kept the pool chlorinator and clean with both the chlorine pills as well as using a vacuum on a regular basis. We also have a gray filter that has a huge amount of recirculation. Does anyone recognize these spots? It is a galvanized stock tank so it shouldn’t rust but it looks like rust. Rubbing it with your fingers and a cloth does nothing. Any suggestions are greatly appreciated. 
