I tried various tank cleaning chemicals with some slight improvement but not total success. I've also used the San-T-Flush without it making much of a difference. The black tank indicators would maybe go from 2/3 to 1/3 full when the tank was actually empty.
Recently, though, my toilet had a pile of toilet paper visible down below the flush valve. I worked it loose with a stick and then dumped several 5-gallon buckets of water down it. I poured the water as quickly as possible to thoroughly flush it out. Now the indicators are working great.
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