[Delphinus]

What media is it and how old, do you know?

If the media was sitting wet I'd give it a good bucket flush before using, but otherwise just a good few rinses to get rid of most of the powder (the media breaks down over time). Assuming it's still usable though - I've found it's generally best to replace media (if it doesn't get completely used up) after about a year. If you compare old media to new media, the new stuff will have a lot more "granular uniformity" (is that a word?) and the older stuff will feel "soft". In an upflow/fluidized reactor it will probably get used up more but in a downflow reactor the mixed grain size will lead to compaction making it harder for the pump to pump through (and you may get cavitation in that case). I'm not sure if old media can leach more phosphate than new media but that might be something to watch out for.

I don't think you get those problems with the Schuran style media now available (much larger pieces). I haven't tried that yet as I'm sitting on a few containers of ARM I figure I should use up first before switching. I think for those you need to run your reactor at a much lower pH though.