Fritz RPM Liquid PT 3 Magnesium | Reef Magnesium Supplement for SPS, LPS & Soft Corals

Fritz RPM Liquid PT 3 Magnesium is the magnesium component of the Fritz Reef Pro Mineral dosing system. Stable magnesium (Mg) levels keep calcium and alkalinity from precipitating out of solution, which is why Mg is the unsung hero of every successful reef tank. Pharmaceutical-grade and made in the USA, PT 3 delivers a clean, predictable boost any time Mg trends below the reef-safe range.

Dose alongside RPM PT 1 (Alkalinity) and PT 2 (Calcium) for full reef chemistry support, and verify levels with a precision test kit like the Salifert Magnesium Test Kit or Hanna Marine Magnesium Checker HC. Browse the full additives & supplements collection.

Why Magnesium Matters

Magnesium acts as the buffer that keeps calcium and carbonate ions in solution. When Mg drops below ~1250 ppm, calcium and alkalinity become unstable and start to precipitate out as scale — turning your tank glass cloudy and starving corals of the minerals they need to grow. Most reef hobbyists target 1280–1380 ppm magnesium; salt mixes alone often fall short, especially in tanks with heavy SPS growth.

Features — All Sizes

Choose Your Size — Variant Comparison

Select a size in the tab section below for the full individual spec sheet.

Detailed Variant Specifications

Click a size below to view its full specification sheet.

Which Size Is Right for You?

16 oz — Best for pico/nano reefs (≤ 20 gal) or correcting a single low reading.

32 oz — The everyday size for small reefs in the 20–60 gallon range.

1 Gallon — Best value for mid-reef hobbyists running ongoing Mg supplementation.

5 Gallon — The choice for large reefs and SPS-dominant systems with high Mg consumption.

How to Dose

Test your magnesium first. If your Mg is below 1280 ppm, dose no more than 100 ppm of correction per day to avoid stressing livestock. Spread the dose throughout the day, ideally in a high-flow area of the sump or display. Retest 24 hours after dosing before applying another correction. For ongoing maintenance, dose daily based on your tank's measured Mg consumption rate.

FAQ

What Mg level should I target?
Most reef tanks do well at 1280–1380 ppm. SPS-dominant tanks often run slightly higher (1350–1400) for stability margin.

Why does my Mg keep dropping?
Heavy coral growth, calcium reactor use, frequent dosing of Ca and Alk, and even some salt mixes can pull Mg down over time. Test weekly and dose to maintain.

Can I dose PT 3 at the same time as PT 1 and PT 2?
Yes — unlike Ca and Alk, magnesium can be dosed alongside either without precipitation issues. Many hobbyists dose Mg first, then Alk, then Ca on separate dosing pumps.

Is it safe for refugiums, inverts, and macroalgae?
Yes. Mg supplementation at the recommended rate is reef-safe and supports invertebrate shell/skeleton health.

Do I still need this if I do regular water changes?
For lightly stocked tanks with frequent large water changes, water changes alone can keep Mg in range. Most tanks with significant coral growth need supplementation between changes.