IME, 9/10 problems have been related to flow in one way or another. Not necessarily fish deaths but some yes.
No way those fish in a 33g could get the oxygen they needed if the only flow source was blocked. When oxygen deprivation occurs, the fish that needs it the most expires(dies) first. This frees up a little O2 but then ammonia kicks in. IMHO, if it wasn't a cycling tank then this deadly combination killed off your fish one by one.
Sorry for your losses