10% of 30gallons is 3 gallons
5% is 1.5 gallons
After the inital dose of salt (a 30g tank would be 6 tablspoons) an raising the temp to 84 for 7 days its not neccessary to replace it if all is well.
I personaly keep salt in my tanks at a rate of 1 tablespoon per 10 gallons as a preventive cure.
BY the sounds your red devil has been sick for a while so I would reccomend keeping salt in the tank for a few weeks at least.
With a 30g I would suggest water changes at a rate of 5 gallons "Twice" a week to keep the red devil well.
I dont reccomend anymore fish in with the red devil which would add to his problem in regards to overcrowding,stress from bio overload etc.
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TONY
Australian and proud of it!! My Cichlidfish.com Home pageNo Feeder Naziworldcichlids.com