Okay. The facts so far 50% water change with tapwater. That's a pretty big water change. You probably have a bacteria bloom from the declored tapwater due to the fact that Edmonton's tapwater is treated chloramine. Chloramine is a compound consisting of ammonia and chlorine, with about 1 ppm ammonia.
We had a recent discussion on chloramine in Edmonton's tapwater here....
http://www.canreef.com/vbulletin/sho...light=ammonium
Instead of waiting for a bigger tank, you should invest in an RO or RO/Di unit, or else use RO or distilled water from the grocery store for water changes.