[DJRansome]

The reason you keep nitrates between 10ppm and 20ppm is that the fish cannot tolerate sudden, large changes in the levels, even if the water is getting cleaner. What salt are you adding?

If you have a tank that has gotten very high over time (no rust does not cause nitrates...they are organic) then you do smaller more frequent water changes so the change is gradual.

I like to do 20% daily or 2X daily when the tank has very high nitrates. Tangs are more sensitive to water issues than Malawi and cyps are THE most sensitive I have ever kept.

Metro does not last more than 24 hours so go ahead and change water to keep it under 20ppm (unless it is already much higher than 20ppm).

 

[DJRansome]

If nitrite is zero there is no need for a water change. Has the level of aquarium salt been exactly the same since you started the tank?

I don't use garlic at all so you are on your own there.

When I use metronidazole I dose 2X daily.

 

[DJRansome]

All fish are sensitive to large parameter changes of any kind.

Malawi will survive things that Tanganyikans will not, but either can die.