Reddit reviews ASG Ultrair 12-Gram CO2 Lubrication Cartridges, 5ct

You may have a blown seal or stuck valve.

1. Regularly oil it. EVERY TIME you shoot it you should put a drop of pellgun oil on top of the CO2 cartridge. They also make "lubrication cartridges" which are special cartridges that have oil in them. Good to use one of those every few thousand rounds.
   
   
2. DO NOT leave an unspent (but pierced) CO2 after you finish shooting. That can damage the seals. Unscrew it and release any extra gas when you're done.
   
   So, I would try one of the lubricating cartridges and see if that fixes it. If not you'd have to take it apart and see what's happening. I have only seen this happen to guns that are not routinely maintained (other than defective). The third option is that it's a lower end Umarex, and they definitely break/malfunction sometimes.
   
   EDIT: They sell Crosman Pellgunoil at Walmart. You will probably need to buy the lubrication cartridges online.
   
   Pellgunoil
   
   Cartridges
   
   

It's a lubricant used to help ensure seals on paintball guns fire properly. As far as I can tell, these are specially marked canisters - https://www.amazon.com/ASG-Ultrair-12-Gram-Lubrication-Cartridges/dp/B00CYGV9BW

Decades ago CO2 gas came in different grades - Food grade and industrial grade. Industrial was lower grade used for other processes where volume was more important than purity. Industrial-grade CO2 is a thing of the past, due to the manufacturing process it's all the same purity these days.

That said the OP asked

>Is it true that it's not safe to use paintball canister

And the answer to that from what I can tell is no. Because the interior of the tank has not been properly lined or sanitized prior to filling.