I would change the medium to perlite or a mixture of perlite and vermiculite, make sure the wick is reaching the bottom of the reservoir and the top of the wick is long enough to wet up the medium.

You can mix sand and perlite/vermiculite in 1:1 volumes to give the plant some support. You can also use sawdust with sand.

Forgot to mention:
Check out "New to Hydroponics", to view some pics on this system.

Thanks, I checked it out. How much risk do you run of over-watering using the perlite and wick system? That's always been a problem for me, which is one major reason why I switched to hydroponics.

If you look at the design, then you'll see that the bottom of the inner pot stays well above the bottom of the outer pot. The wick is there to cover the distance when the nutrient goes low.
You can figure out the distance that would suit you best.

One problem with this is the algae growth over time. These days I cover the units with a piece of plastic pipe. Works well. Also, sawdust tends to go acid very quick. Be careful!

Have fun!

Try putting them outside in the sun for shorter periods each day, maybe 15 minutes to start, and gradually extending the exposure. They may need time to acclimate.

They seem to be doing okay in the shade all day now. Maybe the root system wasn't developed enough before. In a week I'll start putting them out in direct sun for half an hour at a time. I also need to find an outer pot to shield the bottles in.

I've got them in very weak nutrient solution at the moment. Should I wait until they're used to full sun before I up the concentration? Basil shouldn't need a lot of fertilizer, is that correct?

Once basil starts to grow, you can start off at ± 50% strength, gowing up till ± 100% when fully grown. Max EC value = 1800. Keep the pH between 5.5 and 6.5.

How's Hong Kong these days? Used to come there every year for business. Last time 10 years ago. I'm retired now. Miss that place terribly!

I'm afraid I'm too low-tech and poor for all the equipment to measure EC and pH (which is why I'm growing things in soda bottles), but thanks for the advice. The hydroculture nutrient solution I managed to obtain is a liquid, and the bottle carries no information other than to use 10 ml per 100 ml of water for houseplants and 20 ml per 500 ml for bulbs. Not very helpful, I know. The brand is Plantmate.

I've been having a slight bug problem, so I've been misting my plants with mild soap solution. Some of it's gotten on the aggregate, and I hope it hasn't turned it too alkaline. Perhaps I should leave them out in the rain for a bit to flush them through?

Hong Kong's doing fine, by the way, aside from the air pollution which has gotten worse in the last 10 years. And the buildings keep getting taller and they're filling in more of the harbour, but that's another story...

Being poor is not a problem. It's not an exact science. From what you tell me I would say that 10ml per 100ml for houseplants is about right for basil. When they start to grow again (new leaves) start off at ±5ml per 100ml. pH? You know someone who can help you? Someone with a fishtank perhaps? You can buy these pH-sets for fishtanks. They're not expensive.

Personally, I am interested in experimental hydro at the (very) low end of technology. I'm interested in passive systems, and how to make hydro-nutrients from base materials (grass, urine, compost, etc).

Keep in touch, and dont let them fill up the harbour! Used to hearl my heart out on the crossing!