You bought rock that was leaching phosphates then as a tank shouldn't have .1ppm phosphate levels unless you brought it in on the rock or are overfeeding..
Continue with GFO and water changes to lower phosphate amounts and give it time..
5 months still new..
I always say plan on issues for the first 8-12 months of a tank as it takes time for it to mature..
What are your nitrate levels?
Hey MAC, So I received my Hana CHecker ULR.. and my first readings came out as 200.. which = .6132 phosphates.. I'm the guy still battling the GHA.. I manual removed some couple days ago with a 3 day blackout...getting my GFO delivered tomorrow.. I do believe my rocks are leaching.. will just keep fighting.. wonder if it;s high atm cuz I brushed gha couple days ago. I'm gonna test tonight to see if it dropped. My methods to lowering phates are 10% weekly water changes, GFO tomorrow, and NOPOX ( but only 4ml a day) should I up this dosage?