How to get rid of the brown stain from inside the toilet tank?

Drain it again.

1. Soak in 1 gal of bleach plus water to go past the line for at least 24 hours, then flush to rinse after you turn the water back on.

OR.

2. Paint the inside with white waterproof enamel. Just make sure the toilet works properly and is very clean. ☺️

Try dental tablets for the cleaning of dentures

Also you can use a pumice stone to scrub the porcelain, it won't damage the porcelain. Here is a step by step article: https://www.google.com/amp/s/m.wikihow.com/Use-a-Pumice-Stone%3famp=1

try plain coke

Any cola pop will do. Let it sit there for half a hour and then flush.

as strange as this sounds...pour a can of Coke, not diet, into your tank, swish it around a little and let it set for 30 minutes or more. The contents of the Coke should clean it off...it will also get rid of rust on your metal car bumpers...Thirsty yet? Ha Ha.

If your tank has mineral deposits, it will turn it dark yellow or brown. Pour at least 2 cups of white vinegar in the bowl and let it set all night. Then scrub it with a brush in the morning. I do this, and sometimes I use the Mr. Clean Magic toilet scrubber and use it after I do the vinegar treatment. I have no trouble keeping our minerals out.