HYDROS pH 4.0 Calibration Fluid is a single-use, high-precision pH 4.01 reference solution for calibrating aquarium pH probes and controllers. pH probes need two reference points to read accurately, and 4.01 is the low-end reference — used together with pH 7.0 whenever you're monitoring lower-pH water, such as a calcium reactor on a saltwater tank or most freshwater aquariums (which typically sit below 7.0 pH). Key details pH value: 4.01 (low-end calibration reference) Best paired with: HYDROS pH 7.0 for two-point calibration of low-pH applications Format: high-precision single-use packet Shelf life: store in a cool, dry place for up to 1 year Frequently asked questions When should I use pH 4.0 calibration fluid? For low-pH monitoring — calcium reactors and most freshwater tanks — paired with pH 7.0. How many calibration solutions do I need? Two, on either side of your target pH. Use 4.0 + 7.0 for low-pH water, or 7.0 + 10.0 for reef and saltwater tanks. Which controllers use it? Any HYDROS controller with a Probe Port, such as the Control X3, X4, or Launch. Shipped fast with expert support. Get your HYDROS gear from real reefers at Marine Concept Supply, with setup help when you need it and local pickup at our Wheeling, IL showroom.
