How can I get a bromine reading in an epsom salt solution density of 1.3?

The test strips are currently giving a bromine reading of zero (we've watched the reading go down to zero as we add the epsom salt). The other readings (pH and Total Alkalinity) seem to be fine, so our best guess is that the magnesium in the epsom salt (magnesium sulfate) is somehow skewing the bromine reading on our test tabs. does any know anything about this, or know of any test kits that can accommodate?

How can I get a bromine reading in an epsom salt solution density of 1.3?