Quick update:
Switching to Prime and using the higher dosage has sorted things! Thanks to all of you for the advice
One person asked about the age of the tank: It has been running for about 12months.
Would love to get recommendations for best canister filter media.
Also, the tank has consistently low pH. While the fish seem just fine, I'd like to raise it a bit...would a small bag of crushed coral in the filter be the best way to raise it?
thanks again!