All 0 isn't good unless you have a heavily planted tank, there should always be some nitrate in the water if your tank is cycled correctly. Readings of all 0 either mean your tank is planted to the high heavens and you're very lucky (although there'd usually still be some reading of nitrate) or your tank isn't cycled. If it's the latter, that's probably why your fish died.
If you're doing only fortnightly changes with two plecs, that's probably not the best regime either. Especially in a small tank. Even if the water's ok, the poo on the bottom will build up - and cause a build up of bacteria (columnaris is often found in the gravel apparently), and where do your bottom feeders spend all their time? On the bottom.
IMO you need to gravel vac once a week, and as you're doing that a water change wont harm either. But if your tank isn't cycled that's what you need to deal with first.
How long has the tank been set up, how long was it before you added fish, how did you cycle it, and is it planted now?