Cal hypo can be used alone, IF

1. You have a sand filter;
2. You have no other feeder system in use (SWCG, trichlor tab feeder, etc.)
AND
3. You follow some very specific directions I provide.

I've operated large indoor commercial pools that way, and literally used no other chemicals for an entire season, while maintaining proper water balance. On outdoor pools, you will either have to use some dichlor OR add stabilizer manually, and you may occasionally need a bit of acid. But, this method is potentially dangerous, if you don't follow instructions exactly.