It's normal - they seem to restrict airflow a bit and aren't up to quality. Received a couple of these in a 6 pack, the valve part read was poorly placed or not fully connected. With the other devices, I've found that they restrict airflow a little more than other check valves I've used.
KEDSUM 330GPH Ultra Quiet Submersible Pump With High Lift And Grounded Power Cord - Ideal For Fish Tanks, Ponds, Aquariums, Statuary, Hydroponics - Includes 3 Nozzles And 1500L/H Water Flow
39 Review
Sink filter Prio New Water Start Osmos OU380 white
79 Review
Humidifier / air purifier "Fanline Aqua VE400" with an ionizer, with a capacity of 410 gr/h
44 Review
Aquael Fan Filter Mikro Plus
35 Review
Steam cleaner Deerma Deerma DEM-ZQ610 EU, white/red
68 Review
𧲠Efficient Magnetic Fish Tank Cleaner: KEDSUM Floating Clean Brush with Handle Design
34 Review
Mini pump Xiaomi Mijia Electric Pump 1S, plastic, black
55 Review
π U-BCOO Aquarium Gravel Cleaner Fish Tank: Manual Siphon for Efficient Filter Gravel Cleaning and Water Change in Tanks
22 Review