The easiest approach, if you want to use chlorine, is to initially start out using Dichlor until you build up the CYA level to around 30 ppm or so (around 33 ppm cumulative FC using Dichlor) and then switch to using bleach. That way, you don't have an overly high effective chlorine level from using only bleach while also avoiding the continued buildup of CYA from using only Dichlor. Usually this means around a week or two of Dichlor usage if your daily chlorine use is around 4 ppm FC per day (in your case it'll be more like 4 days of 9 ppm FC daily usage). If you initially shock your hot tub after a refill, then doing so with Dichlor could get you to the CYA level you want faster.
If you only used bleach, then the chlorine would be too strong and will wear on your hot tub cover faster as well as your swimsuits, skin and hair. It will also tend to outgas faster so the usage would be higher and it would smell more.
If you only used Dichlor, then the CYA would build up. For every 10 ppm Free Chlorine (FC), Dichlor also increases Cyanuric acid (CYA) by 9 ppm. After a month or so in most spas (a few weeks in your case), the effective chlorine level may not be enough to prevent hot tub itch bacteria from growing.
This post goes into more detail about the Dichlor-then-bleach method. You also don't want your Calcium Hardness (CH) to get too high in order to prevent scaling. Usually, 100-150 ppm is about right for spas and will also inhibit foaming. You'll want to keep the TA lower, especially after you switch from Dichlor to bleach, since this will reduce the tendency for the pH to rise. Using 50 ppm Borates in the spa also helps keep the pH more stable.
The rough rule of chlorine usage (without an ozonator) is that it takes 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% bleach or 7 teaspoons of non-chlorine shock (MPS) for every person-hour in a hot (100-104F) tub. So two people for 30 minutes is one person-hour. So these amounts are roughly what you would expect to use, though you need to test the FC level and adjust so that you end up with at least a small FC residual (1 ppm or so minimum) the next time they soak. Initially, you'll need to test every day to get a feel for your spa's chlorine demand, but if it's consistent then you might be able to test a little less frequently, but probably not less than twice a week (mostly just for chlorine level) if the spa is used every day.
You will need to figure out if you want them to soak with chlorine levels up or not. It's technically the safest and most sanitary if they do, but it will smell of chlorine. Most people soak with minimal chlorine and then add chlorine right after they get out of the tub. So long as you still measure a chlorine residual before the next soak, this is an OK approach. It just means that there is a small risk of person-to-person transmission during the soak, but since you're family anyway this isn't a big deal (and I'm pretty sure that if your wife or daughter had some sort of diarrhea illness that they wouldn't use the tub).
Because your 270 gallon spa is relatively small, the amount of chlorine you add after each soak (2 people for 30 minutes) will be around 9 ppm FC. Since this would normally have the pH swing quite a bit, I strongly suggest you use the 50 ppm Borates. Gentle Spa is one such pH balanced borates product. Getting Boric Acid directly from The Chemistry Store or other source is another option. 50 ppm borates in 270 gallons is around 10 ounces weight of Boric Acid (and probably of Gentle Spa).
As for how often you need to replace the water, the normal rule is Water Replacement Interval (WRI) = (1/3) x (spa size in gallons) / (number of bathers per day) where I believe they assume 20 minute soaks. So that would be (1/3) x (270) / (2x30/20) = 30 days, but most people using the Dichor-then-bleach method find they can go at least twice as long between water changes so around 2 months in your case. Based on your anticipated chlorine usage, your salt level will increase by around 430 ppm per month so you could probably go even longer than 2 months, but as John pointed out doing a water change is a very easy way to reset your water chemistry and start fresh and clean. If you go more than 1 month before changing your water, I'd retest the CYA level and use more Dichlor briefly as needed if it's too low. The CYA may degrade faster in a hot tub.
Your ozonator might complicate things. It can either increase or decrease the chlorine usage. In your case with a higher bather load and smaller tub it might decrease the chlorine usage, which would be fine, but is something you'll just have to see. The ozonator bubbling will tend to make the pH rise more quickly in the spa. If you have a way of turning off the ozonator (including the air injection), you might try that for a while and see which way you want to go (i.e. with or without the ozonator).
Richard
