How soon to get new fish?

I added my last couple of fish about a week ago and I noticed a slight rise of ammonia and nitrite, both to about .25 for a day then a quick drop back to zero. I'm wondering if the spike was more due to me changing some filter media than the fish adding bio load.
How soon is too soon to add another fish?

 

What size tank? What other livestock? How old is the tank? and how much live rock do you have?

 

I would think in a tank that big, with such a small bioload, the spike wouldn't be from the fish. In my 90 gallon I added 4 fish at one time and never saw an ammonia spike. I can't say for sure, but that would be my guess.

 

Tank's the 100 gallon with the fish in my sig. It's been up for about 5 months now. Has a little over 100 pounds of LR, a pretty big external filter in addition to the built in sump, and a Coralife 125 Super Skimmer.

I bet it was probably from the carbon I changed. It was in there for way too long and I needed to change it out but was broke for a little bit so had to hold off til recently. My guess was that it got colonized by bacteria and when the bag was removed it ended up causing a slight spike as the bacteria restabilized. I'm not too worried about it cause it's back to normal now, I just want to add another fish but don't want to cause something to happen to make the tank crash by adding too soon.