Yup, you're right, it is a nitrogen toxicity but I was puzzled as to how this occured because between the Grow Big and the Fish Shit, you're really not giving it too much nitrogen at this stage of growth... but then I saw how much you were watering them and the answer came... You're growing in a 5 gallon and only giving them 1/4 gallon of water/nutes a day - all the nutes are stacking up in the soil and THIS is what is causing the problem. When you give plain water, you need to make sure you get at least 10-20% runoff... this clears out the soil of excess nutes and salts (not ALL but just the excesses that are building up). In a 5 gallon pot, I would be giving them double the liquid you are giving them per day. I also, whether watering or feeding, put a saucer underneath to catch the runoff and will let the pot sit in the runoff for a good 15-20 minutes - that excess water or nute solution is usually soaked right back up into the soil and if it is, I give the plant some more... I keep doing that until the liquid is no longer being soaked back up and then dump what's in the saucer because you do NOT want wet feet! Make sure you let the soil dry out completely before giving anything more.
What you need to do is to clean out the soil with a good flush.... for your pots, that means 15 gallons of pH'd water through each one with 1/2strength nutes in the final gallon... then let it dry out before watering them again ... This should take care of the issue and those lovely lookingi plants should be much happier!
Good luck!