Sorry about your fish. Velvet is harder to see but can be seen if you look. It looks like the fish is covered in small white dots, smaller than ich. They appear dull or dusted has been a description. Your symptoms do sound more like odinium (velvet) than they do ich. Odinium kills in hours to days. Ich can take a week or more. It is also possible that what you saw was a secondary bacterial infection and not the odinium. It is very hard to see and takes a trained eye. If you do not quarantine your fish then you must learn to spot it. I will not purchase a fish unless I have watched him for a very long time. Any scratching on a rock, however subtle is an early sign. Twisting or flicking oddly is also a sign. Except in angels, they do this naturally.

The poly filter will NOT cause the problem. They are very good water scrubbers. Have you added any fish lately? Ich can show up anytime. A stressed fish can develop ich but odinium has to be introduced.

It's lifespan is much faster than ich also. You can leave your tank fallow for 30 days and they will be dead. Some people believe 15 days is enough but I would error on the longer side. The free swimming stage only lasts 24 hours. They must find a host in that time or they die.