Figure Eight Puffer going Blind !?!

30 Gallon Long Tank
Filter 1 - Marineland Emperor 280
Filter 2 - AquaClear 70
Freshwater (was going to start adding salinity next weekend)
Lightly Moderately Planted
Once a week 50% water change

Dry Ferts Added for Plants
1/4 Tsp-KN03 (Potassium Nitrate) 3x a week
1/16 Tsp-KH2P04 (Mono Potassium Phosphate) 3x a week
1/16 Tsp-K2SO4 (Potassium Sulfate) 3x a week
1/16Tsp-Trace 3x a week
1-2ml-Fe/Iron 3x a week

Tank cycled 1.5 months ago
8x Bumblebee Gobies added 1.5 months ago

First Try - Figure Eight Puffer added 1.25 month ago.
The F8 had some scar and damage and that I didn't notice until I brought him home.
After three days, started bumping into the glass and decoration and seemed blind.
Thought it might be because of the his prior injuries, so I returned him to the LFS.

Second Try - Figure Eight Puffer added 1 week ago.
After 6 days, started doing the same thing and seemed like he was going blind.

0 Ammonia
0 Nitrite
80 Nitrate

Did a heavy (75%) water change today.

One of the dry ferts that I'm adding is KNO3 (Potassium Nitrate). So I'm going to skip it for a week and keep testing for Nitrate to see if it lowers it.

Should I do a second water change tomorrow?

Any suggestions?