Blackwater Tank Cleaning

[650Guy]

Question for you all;

Has anyone ever seen this process used for cleaning your holding tanks? I'm considering doing it but do not want to damage anything. Our boat has 2 simple rules:

#1... Here
#2... On the shore.

That being said my level sensors are getting a little finicky and thought i'd try this before replacing them completely.

Holding Tank Descale Checklist – 72-Gallon System

This checklist details a safe descaling procedure for large marine holding tanks (approximately 72
gallons) using a diluted CLR solution. The goal is to loosen calcium and urine scale without damaging
sensors, seals, or plumbing.

Recommended Fill Level and Mix Ratio
For a 72-gallon tank, fill the tank only one-third to one-half full (≈25–30 gallons) with a diluted CLR
solution. This allows the mixture to contact most internal surfaces without risking overpressure or
prolonged sensor exposure.
Standard Mix: Combine 3 quarts of CLR with 30 gallons of water (approximately a 1:40 ratio).
This mixture effectively removes scale while remaining safe for plastic tanks, hoses, and seals.

Step 1 — Pump Out Completely
Empty the holding tank fully and, if available, rinse once with fresh water.
Step 2 — Add the Diluted CLR Mix
Add the CLR-water mixture via the deck pump-out port or toilet. Avoid filling the tank beyond 50%
capacity.
Step 3 — Soak and Agitate
Let the mixture sit for 30–60 minutes. If possible, operate the boat gently so the solution sloshes inside,
helping it reach tank walls and sensors.
Step 4 — Pump Out and Rinse
Pump out completely, then refill halfway with fresh water and pump out again to flush residual CLR.
Step 5 — Neutralize (Optional)
Pour a solution of 1 cup baking soda mixed in 2 gallons of water into the tank. Let sit 10 minutes, then
pump out again.
Step 6 — Final Rinse and Inspection
Flush once more with clean water. Check that tank level sensors now read accurately and that no odor
or scale remains.

Safety and Maintenance Notes
• Do not fill the entire 72-gallon tank with CLR solution — half full is sufficient.
• Do not allow the CLR mixture to sit overnight.
• Never mix CLR with bleach, enzymes, or any ammonia-based cleaners.
• Avoid soaking rubber seals, duckbill valves, or metal fittings directly in CLR.
• For stubborn buildup, consider marine-safe alternatives such as Rydlyme Marine or Dometic Tank
Cleaner.
Perform this procedure annually or whenever level sensors begin reading inaccurately. Following each
descale, use Dometic or Star Brite holding tank treatments monthly to prevent new deposits.

[kgarguilo]

What I have done on my current boat and past boats with all sizes of black water tank. Dump a quart (whole bottle) of water softener in the tank after a pump out, let it fill up naturally to about 50%, take the boat out to slosh it all around, let it sit in there another day or two, then pump out. I do this about every year, and my sides seem to stay pretty clean. It has never caused any damage that I am aware of yet. For reference, 3 large boats over 24 years span.