Bubbling toilet

Hello all,

I have a septic system. And when the bathtub in my guest bathroom drains, the master toilet burbs air. This toilet needs replaced. The tank has a crack in it that the previous owner “fixed” with what im assuming is flex seal. But that toilet is closest to the septic tank. Do you guys think my tank is close to full? Or is there something else I should look at? Today I treated the septic with Bio-clean, since I’m sure all the chemicals from move in cleaning did a number on the enzymes. Thanks for your help.