Found Out How The Rainbows Are Bruising Their Mouths!!

I have just found out how the rainbow fish are bruising their mouths but how to stop it?

I could hear this thumping noise and turned my head and they were attacking the cross bar of the tank!!

Whether there is/was algae there I do not know but have used the algae scrubber on it.

I will check on the cross bar when I do my water change but boy were they giving it a thump. The water level is exactly on the mark so do not know realistically what can be done about it. At least I now know what is causing their lip bruising.