it always affects my hygros first too. it's how i know i've been slacking with my dosing. i think they are more sensative to K deficiency than a lot of other plants.
i don't have any other fertilizer on hand right now, but flourish says that it has 0.37% soluble potassium. could i overdose the flourish to make up the lack of potassium?
No, Because of the levels of the other micronutrients you would run the risk of getting toxic levels of those before you could add enough. K or potassium is the most common deficiency because feeding the fish gives you some of the other two macro nutrients.