a question about metal halide bulbs

[Delphinus]

Sounds sort of like what happens when you use a pulse-start lamp on a probe-start ballast. Do you see any markings on the ballast? If it says M57 then it's a probe-start. If it says M137 or M135 then it's a pulse-start ... odds are *pretty good* though that you have a probe-start ballast.

Another way to tell, probe-start ballasts are a transformer and a capacitor (2 things), pulse-starts are a transformer, capacitor, and an ignitor (3 things). Ignitors and capacitors look similar - usually aluminium - the ignitors might be a bit smaller.

What about the lamps, do you know what brand they are? There's a fair mix of lamps on the market, both pulse-start and probe-start lamps.

Anyhow, without knowing anything more about the setup, that's my guess. If it is a probe-start ballast and pulse-start lamp, it's not the end of the world, they can take up to 20-30 minutes to fire up and stay solidly lit. Usually it gets better after several days worth of use though.

Thing to do though is, when you go to buy new lamps, make sure you order lamps that match the type of ballast you've got. Most vendors will tell you what ballast is needed for the lamps. For example, take a look at this page:
http://www.jlaquatics.com/phpstore/s...t_ID=lb-h1715i

In red text at the bottom of the description it says "this lamp requires a probe-start ballast or electronic ballast." And on this one:
http://www.jlaquatics.com/phpstore/s...ct_ID=lb-h1710
... it says to use a pulse-start.

Hope this helps!