Need advice: Buy new bucket of salt or supplement?

So here is the story. I noticed my parameters decreasing in calcium/alk and I’m using instant ocean salt which I bought a bucket and have started using that instead of a bag. I tested with API and asked on a separate forum and they insisted it I needed to get salifert test kits... I got nearly the same numbers.  I figured maybe I didn’t mix the bucket well enough and it may have had stuff settle, so I tested water prior to mixing the salt and after and it’s been the same. Calcium is around 340 and alk is about 9 on both batches. 

My corals are looking great which is what I’m leaning more toward over numbers. So here are my options and I can’t decide which is a better route? 

-Continue using the salt mix but start supplementing.

-Buy a new bucket of the same mix and hope it’s a better composition.

-Switch salts.

Parameters:

28 gallon - 1 fish, 6 corals (1 SPS) I don’t think my tank has a heavy enough bio load that this these parameters should be a problem and with the size I think all 3 options are viable. 

Salinity 1.025

Temp: 77

Alk: 8.9

Ca: 355

Mg: 1140

Any advice on which is the best/cost effective/least headache approach?