Hang onto your $. As others have commented, look at your tank and evaluate. I've been through the kits and last tried Salifert's nitrate and Hanna's low range phosphate photometer, both great kits imo, but still coming up zero on both and yet bubble algae is present and multiplying. I've upgraded my skimmer and going to see what impact that has (for sure pulling more skimmate than the old one.) If that doesn't do it, on to vinegar additions to my kalk.