help on scales... advices pls!

this is what happened to the aro. there are white spots/bubble like on the aro. it's not the white spots where it should scratching itself. im confident that is a different one. it's on all the scales(both sides). non on the fins at all. nothing on the head or face of the aro. only the scales. i can't come to a conclusion. maybe the picture could give you guys a better picture. i don't know what is the cause. it started 3 days ago. it was resting at the bottom corner of the tank slanting to a side when i started to realise. infact the appetite dropped. but right now as back to normal (appetite). only realised it was there on thursday night.

-swimming posture is as normal.
-water parameters (0 to 0.1 ammonia 0 nitrite).
-ph was 7.3 when i checked. starting to reduced the ph gradually.. now is -somewhere btwn 6.8 to 7.0 .
-water temp ranging from 30-31 degrees.
-diet the past few days was (all in quarintine tanks)
goldfishes (7pc in 2 days)
centipede (1 pc yesterday morning)
feeder frogs ( 2 pc yesterday night)
-tankmates only a small pleco. (pleco is normal)
-tank 4x2x2.
-filter canister + ohf + 2 powerheads (1 to shift the poos to aside of the tank and another 1 to suck it up)
- filter media (no AC. just ceramic balls, rings, bioballs, jap mat, wools, substrate, biohome, and 2 holes beads) 1 kg for each except the CR was 3 kg.
- added 3 pills of antibiotics and added 1% salt (not aquarium salt)
- wc was 35% on saturday night.

on wednesday i added Arowana Vital from Tetra the big yellow bottle. Never overdose, underdose it.

did i missed out anything? please help/advice what i should do next?

type too fast.. i didn't add 1% salt, just about 0.5% salt.