Another vote for Leak Sentinel. I was an early adopter soon after it came out. I can honestly say it is the most worthwhile add-on bit of kit I have ever bought. I use it with an Aquatica system and have upgraded to the Mk 4. Have never had any false positives; it has all operated flawlessly
Like others have commented above, Miso is brilliant in terms of service, assistance and advice - I needed a small spare part recently which he supplied FOC despite my unit being about five years old. Promptly too - two days from Slovenia to UK.. I am very impressed. And don't be put off by any thoughts of a language barrier - his written English is as good as most stuff you see on Wetpixel!