Hanna Instruments HI735 / 735 | Total Hardness Low Range Checker HC | Freshwater GH

A one button digital reader for General Hardness. The Hanna HI735 Checker HC measures total hardness, the combined calcium and magnesium in your water, and shows it as ppm (mg/L) of CaCO₃ on an LCD. That is the same thing aquarists call GH, so the 0 to 350 ppm range covers roughly 0 to 19.6 dGH.

GH matters for osmoregulation in fish, shell and exoskeleton building in shrimp and snails, and mineral uptake in plants. Reagents are sold separately: see the HI735-25 refill pack, or browse all water testing and the rest of the Hanna Instruments range.

How It Works

• Fill the cuvette with a water sample and zero the meter
• Add one reagent packet, cap and shake
• Put the cuvette back and press the button for the reading
• The calcium and magnesium react with the reagent to produce a red violet tint, and the meter reads the color through a 470 nm LED and a silicon photocell
• The method is Hanna's adaptation of EPA Method 130.1

Freshwater and Source Water Only

This is a low range hardness meter. It tops out at 350 ppm, so use it on freshwater aquariums, tap water, well water and RO or RO/DI output. It will not read saltwater or reef tank water, where hardness runs far above the top of the range. For a marine tank use a calcium or alkalinity checker instead.

Hanna lists the HI735 for drinking and process water, heating and cooling systems, and pools and spas. The aquarium GH use is the same measurement applied to tank water.

What Is in the Box

• HI735 Checker HC meter
• Two sample cuvettes with caps
• Graduated syringes and a refilling pipette
• Reagent starter kit. Hanna states this covers 12 tests, though at least one overseas distributor lists 23, so treat 12 as the figure to count on
• 1.5V AAA battery
• Instruction manual

No carry case is included. Hanna sells the HI735-11 certified standard kit separately if you want to verify the meter against a known value.

Specifications

Frequently Asked Questions

Is GH the same as total hardness?
Yes. GH is the combined calcium and magnesium content, which is what a total hardness test measures. Aquarists usually quote it in dGH, where 1 dGH is about 17.86 ppm as CaCO₃.

Can I use it on my reef tank?
No. Saltwater hardness is far above this meter's 350 ppm ceiling, so it will simply read over range. Use it on freshwater tanks or on the RO and tap water going into any tank.

Does it come with enough reagent to get started?
Yes, Hanna includes a starter kit of 12 tests. After that you need HI735-25 refills.

Does it measure KH or alkalinity?
No, those are a separate measurement. Hanna makes dedicated alkalinity checkers for freshwater and for marine tanks.