Whooo nelly - 30 secs in the fix? Even the Ilford literature suggests 2-4 mins. I have found that even conventional grain films don't clear in 30 seconds.
I think you need to satisfy yourself and do a clip test. Pour some of you diluted fix into a beaker and cut off around 2 or 3cm's of film (use a waste piece - a leader off of a roll of 35mm will do). Put a small drop of the fix on the centre of your film cutting, wait a few seconds and then drop it in the fix. Observe and time how long it takes for the film to clear. You can tell when its pretty much done, as the drop in the middle and the rest of the film will look the same. From my experience, conventional grain film (FP4, HP5, ect) will take between 30 secs to 45 secs on fresh fix. T/Delta/ non conventional grain films can take up to a minute and a half.
The suggestion is that you should go at least twice the time it takes for the film to clear - but longer doesn't hurt either.
I personally go really long on the fix - 4 to 5 minutes on conventional grain films, 8 to 10 minutes on non conventional films (using 2 bath). This is more a bit of OCD behavior, as opposed to what is necessary - but 30 secs is way to short in anyones language. There is evidence that too long a fixing will start to bleach the highlights - I haven't found this in my experience. I am open to suggestion though!