Jaap, just a note to your last post. If you have a look to your filter instructions you will see that there is a table with some figures for different models (page 6 in English). Fluval 305 does not have an output of 1000 lph, this the pump output. Have a look to the 7th column in which filter circulation is provided. This is the output, 710 lph. However this is probably measured without media and very short tubing (not explained in this document but for example if you have a look to JBL instructions they explain this very well and even give you the real figures, with tubing and all the media). As explained many times in this forum the real figure can be up to 40% less... Let say you have around 400 lph. Now let's assume that this reactor you don't use is restricting the flow by 25% (not sure because I have never used one, but I bet I can be even more)... This give us not more than 300 lph.
Next time you clean the filter just measure it. Take the outlet tube, put it in a bucket and measure which volume is delivered in one minute. Then do the calculations and check what you really have as flow. I'm not saying this is your problem, but just to point out that this reactor is not doing any good there.
Jordi