what supplements?

The major things that get depleted are calcium and alkalinity. There are several ways to supplement these:
-two-part additives, like ESV B-Ionic
-kalkwasser, a saturated solution of calcium hydroxide
and more fancy things like Nielsen reactors and calcium reactors.

The first thing you need to do is get a few decent test kits. If you don't test, it's a bit dangerous to dose. I like SeaChem, but salifert's are also excellent. You may find that your levels stay perfect with just water changes.

Although some say that iodine and strontium may need to be dosed as well, I have not seen a lot of data supporting those claims.
 
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