Would reducing the GH dosing to say 4-5 help ?We are seriously living in a mad world where similar effects (hardness etc.) can be obtained by the fraction of the price.
I'd vote for lack of Iron (by the hardness of water overall and inability to consume enough of Fe at the early stage of growth) covered by Darrel anyway.
There are some Fe ferts which are making Fe available at higher alkalinity (EDDHA maybe?). If you're afraid of shrimp bodies decomposition, they can be kept in rather low GH (2-4-ish) while additional Ca is provided by bits of snails/molluscs shells - but they will raise GH anyway.
i have another 180Liter tank that has 7 GH and 5KH ( due to seiryu rocks ) and I dose with the same APT complete, if it is the Fe at high alkaline water , would it not be an issue in this tank too ? Just curious. 180L tank has more stems compared to this tank too.