What could cause a tubs drain to gurgle when the toilet is flushed?
The gurgling typically happens with a vent pipe blockage. Without a source of air to follow behind the rushing water, a vacuum is created that will result in gurgling. You may be seeing that now if the vent pipe is partially blocked and there isn't enough air to support the faster flow. It may be necessary to snake out the vent pipe from the roof and/or run some water to clear out any debris that has collected inside.

Bubbles in a toilet below the water source (upstairs toilet) may be a sign of a blockage further down the drain. If that's the case, you may need your main drain line snaked and the plumber may need to run a scope down the line to find the location and cause of the blockage. It's possible that the enzyme pushed a clog further down the line, or the slow drains may indicate your septic system has started to backup into the house and needs to be serviced.