Aloha from Hawaii. I am on the Big Island and started an aquaponic system last Nov. There is only one problem and that is the lettuce is all bitter. Some say heat, I say nay. I grew up in Iowa where it is 90-100 in summer and never had bitter lettuce. Some say it is growing too slow. I was wondering if a lack of potassium would do it. In checking water this is the only thing not showing up. What do you guys think?