i personally believe MELAFIX to act moreorless as the "essential" oils of the fish keeping hobby and i dont think atleast ime to have any effects on the fish.
I would recommend checking nitrite and nitrate along with the ammonia tests as most fish should be able to self repair their slime coats relatively quickly. and any case of it not happening usually is due to parameters that are disallowing such repairs or internal issues causing the fish's body to allocate more resources to fighting off a parasite than repairing slime coat.
it also seems to be a bit more than just slime coat that is falling apart, there seems to be a burning effect that runs down the back and along the front face flap of the arowana, causing discoloration. this seems to be an issue that stems a bit further from just parameter imbalances.
regardless of when you get a test kit, i agree with
danotaylor
and would highly recommend a 70% atleast WC right now with a double dose of prime. WC are the safest and easiest ways you can improve water quality and maintain fish health.
additionally check your filter to make sure there isnt an eldritch monster hiding inside it, your N-bacteria colony might have all perished depending on the current parameters.
GL on the sitch man, hopefully that aro gets up and running again
Midwater
with telling us about your water parameters and water change schedule.
