Sudden ammonia spike

[Penelope .R]

My tank started smelling horrible early this morning, like I could smell it several rooms over horrible. I spent all day washing new sand (something I've never endeavored to try before) but planned to add it tomorrow. By tonight I had enough of smelling the tank and did a water test with my API master test kit to troubleshoot what was up. The ammonia vial turned dark green, the darkest I've ever seen it.
I find this very weird for two reasons. One, I did a seventy percent water change three days ago. And two, I tested the water Thursday night and the water parameters were near perfect (my ph was lower than I'd like it to be, but that's pretty normal in this tank).
I immediately did a large (90% ish) water change and added the sand then because I could. I tested the water after and the ammonia/nitrite levels were zero, but my ph was much higher than before, about an 8.0 (usually a water change brings it to a 7.0). I used a ph regulator to bring it down to 7.0 and put the fish back in. All of the fish seem fine and I think they like the sand.
What could have caused this ammonia spike?
I don't think my tank is overstocked, I have four silver dollars (two adults and two juveniles), one kissing fish, four dwarf gouramis, three mollys, one pleco and three snails in a sixty gallon tank.
The only thing out of the ordinary that happened is my roommate accidentally dropped in three alge crisps instead of one on Thursday, but the fish swarmed them (my silver dollars love alge pellets, it's like a treat for them). Otherwise I don't think I've been overfeeding them (they are on an all flake and pellet diet for now, although I'm planning on finding some better alternatives soon).
I did just get a new filter about a month ago, a Hydor Professional 250 canister filter. I love this filter, but I didn't put any media from the old filter into the new one (it was just nasty, the fish were barely alive when I came into possession of the tank). But still, I feel like after a month running with no issues an uncycled filter wouldn't be the cause of my problems.
Anyway, I'm just wondering if someone could help me figure this out. I'm not overly concerned, I'm just doing more frequent tests and water changes in the meantime, as well as watching how much food I give them, but some suggestions for maintaining this tank would be nice, it takes a long time filling it with pots and pitchers.

 

[Lajos_Detari]

Some possible reasons for high ammonia/stinking tank:
1)Having dead fish in the tank
2)Plants are dying
3)Filter malfunction
4)Too much leftover food.
5)Something in the water(chemical, etc) that killed the beneficial bacteria in your filter.

 

[madmark285]

I did just get a new filter about a month ago, a Hydor Professional 250 canister filter. I love this filter, but I didn't put any media from the old filter into the new one ......
but some suggestions for maintaining this tank would be nice, it takes a long time filling it with pots and pitchers.

Not transferring some of the old media to the new filter was a mistake, it will take a ~month to cycle the new canister filter and you such expect ammonia and nitrites spikes in your tank.

For maintenance, get a 5 gallon bucket or buy a python water change system. Using pots and pitchers would be a bit frustrating for a 60 gallon tank.

 

[Penelope .R]

Not transferring some of the old media to the new filter was a mistake, it will take a ~month to cycle the new canister filter and you such expect ammonia and nitrites spikes in your tank.

For maintenance, get a 5 gallon bucket or buy a python water change system. Using pots and pitchers would be a bit frustrating for a 60 gallon tank.

Thank you for the reply.
Thinking about it now, I did put the old media in the new filter. I put some of the old sponge on top of the new sponge. A lot of it just disintegrated one day when I went to clean the filter and I'd been wanting to get something other than a hang on the back for multiple reasons, so I just bought the canister. I got it up and running December 28th so it has been about a month. I've been doing a 60% water change once a week since and water testing twice a week with no problems.
I just don't get how it could happen so fast. I'm new to this so maybe my understanding of how the cycle works is skewed, I just thought it would be running smoothly by now.