One evening I saw my BGK attacking my ground probe. So I got my multimeter and found that the water has fluctuating current between 100-300 micro volts. Then I got my screwdriver/voltage tester and sure enough it lit up when it touched the water. So I started unplugging everything and found that the Grech canister was the culprit. I switched off the UV lamp and isolated that to be the source. I found that the UV glass was entirely flooded with water. The bulb including the light socket crumbled when I removed the bulb. The contacts were all rusted too and all that was left when I was done cleaning all the mess were the 2 protruding wires from the base. Luckily I found a replacement canister head from Amazon and the estimated arrival was pretty quick. Amazon.com : SUNSUN HW-304B Aquarium Filter Head - Pump : Pet Supplies When it arrived, I found that it came without the UV glass tube so I re-used the old one. I assembled the filter, installed, primed, and heard the impeller running. I noticed that the water was not moving at all. I was pretty sure the filter was properly primed. I even installed it half empty and watch the water go through the filter and back up the tank after priming. Something was wrong and began to suspect if there's an air pocket that would not come out due to some difference in the internal design. So I decided to do some disassembly and inspection to both the Grech and Sunsun filter heads. Here's what I found: - The UV ballasts and motor of both filters are completely encased in epoxy and therefore waterproof. - Water coming from the impeller chamber going back to the aquarium is designed the same way with both filters. Grech GW-304b: - The water inlet coming from the aquarium enters the UV chamber high so there's a greater chance of the water entering the top of the UV tube and flooding it in case both o-rings fail. This is what happened to mine. - There is no anti-flowback so it's harder to prime using the priming button. Sunsun (actually Sensen) HW-304b: - The water inlet coming from the aquarium first enters the anti-flowback chamber. This chamber is where the priming button is located and has 2 plastic doors which swing unidirectionally. This helps tremendously during priming. I don't think it restricts flow that much since the doors are very light and swings quite easily. The chambers are pretty spacious too. - After the anti-flowback chamber, water then goes to the UV chamber low, avoiding the top of the UV tube. Quite a nice design and I was impressed. So I found nothing wrong with the new filter head. I put everything back together and tried again. Same results. I almost thought of returning the filter head and used my old impeller as a last resort. It worked! The new impeller must have been assembled wrong with the magnet upside down. I must say I am impressed with the Sunsun (Sensen) design. Here I am thinking all these years that they are all the same in every way. I plan to repair my old filter head and keep it as a spare just in case something else happens. A spare glass tube and 2 o-rings for the UV chamber just arrived. I'm now just waiting for the G23 2-pin socket to arrive and thinking of a way to affix it to the old filter head. I could not find an identical G23 socket so this new one I may have to modify a bit.