New to spas, very familiar with pool chemistry.
The question is, should a spa be treated in the same way, following the LSI (CSI) saturation index, same CYA / FC ratio's etc.
Are there any special considerations?
CSI does indeed apply to spas with plaster surfaces. Since they typically share water with the pool, chemistry would be the same. Spa water isolated by valves or programmed automation do require minimum water circulation time to ensure good chlorination.