Just in case this help anyone else. The flow on my Tetratec EX600 didn’t seem very good so I thought I’d carry out routine maintenance. It’s loaded with foam, Sera Siporax and a bag of activated carbon. After rinsing the media I decided to change the carbon. The old stuff was quite fine and I had it in a mesh bag. The new stuff from Allpond Solutions has a larger pellet size and comes in its own mesh bag. I put the bag of carbon in the filter but it was a tight fit so I decided to cut it open to remove some. I then decided to just to tip the carbon into the filter without the bag. This will prevent water bypassing the carbon around the edges of the bag where there were inevitably some gaps. There is a top grill in the canister to stop media being sucked up but there are some large holes in it so I needed to cover them with some mesh of some sort. I then though of this stuff pictured below which I’ve used before. Once back up and running the flow of the filter is now much better than before and I’m convinced it was the fine mesh bag the carbon was in that was the culprit. I’ll have to see how it is down the line but for now it’s running well.