Something I do to prevent this happening is the use of these pads...
Activated filter media for all filters, just cut to size
www.pro-shrimp.co.uk
One of my internal filters has a double can setup, so is pretty much identical as an external canister in that I can mix/match various media. I first cut the pad into 1" wide strips then into triangles. They go randomly into the bottom can above the ceramic ring layer...floating bed style. The second can on top has a similar media style using cut up misshaped pieces of anti ammonia pad (again Superfish) with a topping of floss.
(Superfish also do Nitrate and Carbon pads too)
I find using the pads rather than the liquid additives lasts alot longer and is therefore effective longer, since the liquids get removed/diluted each water change. I change the media monthly (floss weekly)
One pad lasts me around 6 to 8 months and when inside the can, it doesn't lose effectiveness...thus saves money buying the equivalent liquid anti phospate which gets diluted over time with each water change