You dumped the tank. You rinsed the tank. The monitor panel still glows "FULL." You're not crazy, and your sensors aren't broken — you almost certainly have struvite.
The Short Version
Struvite is a hard, crystal-like mineral scale (its technical name is magnesium ammonium phosphate) that forms inside RV holding tanks over time. It coats the tank walls and — most annoyingly — builds up on the metal sensor probes that tell your monitor panel how full the tank is.
Once a sensor probe is crusted over with struvite, it "sees" that crust instead of the empty space around it, so it keeps reporting the tank as full. That's the classic symptom.
Why It Forms
Human waste is naturally rich in ammonia, phosphates, and minerals. Sitting in a warm tank, those compounds crystallize onto any surface they can find — especially sensor probes and the lower walls of the tank. The less water you use and the longer waste sits, the faster struvite grows.
Why Your Usual Treatment Won't Touch It
Here's the key thing most people don't know: the enzyme and bacteria treatments you pour in to control odor and break down waste do nothing to struvite. Struvite is a mineral, not organic waste. Removing it takes a descaling process — and a stubborn buildup usually takes more than an off-the-shelf bottle can deliver.
How We Fix It
Our tank service is built for exactly this. Using 1,800 PSI hydro-jetting combined with proper descaling, we strip the struvite off the tank walls and sensor probes and flush it out — restoring accurate sensor readings and a clean tank. It's done at your location, usually in one visit, with no guesswork and no damage to your system.
Keeping It From Coming Back
- Use plenty of water — struvite loves a dry, concentrated tank.
- Don't leave the black valve open at full hookups.
- Run a periodic deep rinse or descaling treatment.
- If sensors start reading wrong again, address it early — it's much easier before it hardens.