Take careful note of the instructions and part 3. about the foaming...........

Reading the bottle instructions would have shown you part 3 as well.
Dosage:
One 33.9oz (1L) bottle for spas up to 1,000 gallons (3,800L)
How to Apply:
1. Remove cartridge filter elements
2. With spa water temperature at 95 degrees or higher, add entire contents of 33.9oz (1L)
Spa Purge to spa water
3. Run jets a minimum of 5 minutes
** If exessive foaming occurs, turn jets OFF
4. Maintain water temperature and allow spa to circulate overnight with jets off
5. Drain spa, rinse and then refill spa and reinstall filter elements. Proceed with
normal spa maintenance
Tips:
For a more complete spa cleaning, soak cartridge filter elements overnight in the
only all natural filter cleaner: Filter Perfect
To prevent future build up of organics, use Natural Chemistry�s Spa Perfect weekly
Features & Benefits:
Attacks the root cause of maintenance problems caused by organic contamination
Highly concentrated formula
Restores spa plumbing to original condition
Removes organic waste deposits from all spa plumbing

The best way I know is to hyper chlorinate for several hours and then dump the water. Refill, dump again after running a couple minute and refill. Put in new filter elements and balance following the instructions found on either Trouble Free Pools or Pool and Spa Forum (better for spas I've found but TFP is still excelent).

Scott

Specifically you need to use a line flush product to remove any potential bio-film in the plumbing lines...of which there is likely plenty. The best commercial line flush product I have used is made by Aqua Finesse however the Spa Guard brand swirl away pipe flush is also good.

Foaming is an indication of garbage/oils in the water. Be sure to take the filters out of the tub as they do not need to be in while flushing your lines and can deposit detergents and oils in the water which will promote the foaming. Dump the water and refill and THEN you can flush the pipes again which maintaining 10ppm of sanitizer overnight. This will help to get the tub really clean and minimize foaming during the cleaning process.

Also consider reading the Spa School link below and learning about proper hot tub maintenance

Here is a link that might be useful: Spa School

When all else fails read the instructions :~)