Thank you my fellow-aquaponers for the suggestions. The bug-approach (aka buy myself a few dozens lady bugs) was next on my line of attack. But I have my system in a finished basement where we spend quite a bit of time, and the idea of sharing the space with ladybugs wasn't too appealing... although we were going to put a big mosquito net over it to try to contain the bugs. Instead, we tried another approach: we have been spraying the plants with a product called "AquaPest-Away" (bought online). It seems pretty natural (mixture of sesame oil, rosemary oil, geranium oil, peppermint oil, citronella oil and water). At first, we sprayed every other day for about 3 weeks, then we went to about once a week. It's been a month now, and I don't see any thrips anymore. The bottle is a little pricey (about $ 20 for 16 oz), but my system is relatively small. I just bought another bottle and will probably spray the leaves sporadically as a preventive measure. A word of caution: if you don't shake the bottle well before spraying, the concentration at the end of the bottle is too strong and will make your plant leaves wilt. Hope it helps!!!