I'd try vinegar.
If you can tip the tank, try letting it pool at full strength. If not, you could possibly fill the tank with a water/vinegar solution (cosco has bulk vinegar for cheap).
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-Mark
29 Gal Bowfront w/24" LED Lights. DIY HOB Sump (5.4 Gal) MP40. Orange Spotted Watchman Goby, 2 Clownfish and a few hermits.
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