We have a small 350gal hot tub. Using the frog floater for chlorine and BB for TA /pH. When I check chlorine using the FT-100 kit, the water typically stays red or goes clear for less than a minute and returns to red. It reads low to normal when using R-0600. A test strip appears normal. Is the DPD just not intended for hot tub water? Is there a different process for tub water vs pool water? Insights? Thanks.