I'm no expert... but I think it's really strange you have a stable thriving tank and then get completely plagued by GHA. The numbers you were maintaining don't jump off the page as OMG he will have issues! I understand what was mentioned above about the rocks absorbing some PO4 and then leaching but then I would ask what about the tanks with extremely high PO4 that are running long term tanks with no issues.
I find it a little worrisome and discouraging this can happen even when you are doing the correct things. I think you were doing everything right. Some changes I think you may have jumped to, too quickly but you corrected it. I have been looking into a product called Vibrant but I do not have any particular reason to use it. It may be worth looking into as many have had reported success.
You have a nice looking tank full of acros though, even with the GHA.
I find it a little worrisome and discouraging this can happen even when you are doing the correct things. I think you were doing everything right. Some changes I think you may have jumped to, too quickly but you corrected it. I have been looking into a product called Vibrant but I do not have any particular reason to use it. It may be worth looking into as many have had reported success.
You have a nice looking tank full of acros though, even with the GHA.