Read this for recommendations on how to maintain a hot tub/spa with chlorine. Now, that is based on a stand alone spa. Maintained properly like a pool extra "stuff" is generally not needed.
From the picture it appears to be in ground, but is that stand alone or connected to a pool? If it is connected to a pool you generally need to exchange water regularly so that the spa is maintained along with the pool.
We do not "shock" our pools, we follow the
SLAM Process when there is a problem. For a small spa like that I would just drain/refill it and start over rather than SLAMing. If it is connected to the pool and you have this problem in both places than a SLAM is in order.
How much
Pool School have you read? Start with these:
ABCs of Water Chemistry
Recommended Pool Chemicals
How to Chlorinate Your Pool