You might need to do more Craig - think about it this way - if you remove half of the water you also remove half of the ammonia. If ammonia is at 0.5 then removing half of it will only reduce it to 0.25 as you have discovered. Levels need to be as close to 0 as possible, so that may mean multiple large changes (like 80-90%). Leave a small amount of water in the bottom - enough for the fish to swim in - and change the rest out. If it's still high, then do the same again immediately. While multiple large changes may stress your fish slightly, it's better than ammonia poisoning. And perhaps cut out feeding completely until your mini spike has finished. As has been said, it should only take a couple of days for your filter to catch up as you've moved all your media over which was dealing with the bioload fine before. How long did it take last time? Also, now may be a good time to start using a hose to do your water changes, unless you already do!