TL;DR

Hard water breakthrough means the resin bed has exhausted before regeneration triggers. Check salt level and brine concentration, then verify the regeneration time clock setpoint against actual water usage.

What you might see

outlet water hardness above zero on a test strip or meter
scale visible on downstream equipment or sample ports
softener regeneration cycle running more frequently than normal
brine tank salt level dropping faster than expected

Likely causes

Tap to mark as ruled out

Salt bridge in the brine tank preventing brine from forming at the correct concentration
Exhausted or fouled resin reducing the effective exchange capacity
Regeneration cycle too infrequent relative to current water demand
Brine injector plugged, reducing the amount of brine drawn into the resin during regeneration

Required tools

Water hardness test strips or colorimetric test kit
Brine tank probing rod
Screwdriver for injector access
Salt (correct grade for your softener)

Safety first

Turn off the water supply to the control valve before removing the injector or any part of the brine system under pressure.
Use caution when handling salt in a confined utility space. Bags are heavy and floors can be slippery from brine spills.

Procedure

7 steps · ~45 min

1
Test the softener outlet water hardness with a test strip or a colorimetric kit. A positive hardness reading confirms breakthrough.[1]
2
Check the brine tank. Use a broom handle or rod to probe for a salt bridge (a hardened salt crust over an air gap). If found, break it up with the rod.
3
Verify the salt level in the brine tank is at least 6 inches above the brine valve. Add salt if low.
4
Initiate a manual regeneration cycle on the offending tank and monitor the brine draw phase. Confirm the system pulls brine for the specified draw time.
5
If the brine draw is short or absent, inspect and clean the brine injector nozzle. Mineral deposits commonly plug the small-bore venturi orifice.
Warning: Turn off the water supply to the softener control valve before removing the injector nozzle. Disassembly under pressure can cause water to spray under high velocity.
6
Review the regeneration frequency setting against actual daily water usage. Increase regeneration frequency or reduce the exchange capacity setpoint if demand has increased.
7
If breakthrough recurs after resolving salt and injector issues, pull a resin sample and submit it for analysis to check for resin fouling or mechanical breakdown.

Sources

Culligan Marlo MGT / MFT Twin Water Softener / DI System general technical documentation, Culligan
Ion exchange water softener operation and troubleshooting, general water treatment references (general)

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