Hi Tammy,
Well that paragraph pretty much explains your problems!
We all argue a bit about where to hold the test tube. Personally, I clamp it right against the white part of the card because that way I can hold it in one hand. I think the more important thing is to be consistent about the lighting and what I'm doing with my other hand is adjusting the little spot lamp so that my reading is on 3 or 4 inches in front of this light, making sure I just do it the same way repeatedly. Repeated measurements that show trends are more powerful than any one measurement.
Unfortunately, the fish-load/filtration thing worked out backward for you! You ended up with "fish before biofilter" and thus find yourself in a "fish-in" cycle. If you find one of Miss Wiggles (or some others have it I think) posts on "fish-in" cycling there will be the techniques to follow.
Basically, because the undergravel filter bacteria either died out, or was at an extremely low level because of there being only one fish, you just do not have an operational biofilter and won't have one for at least about a month after you get the new one and start fish-in cycling it. It takes about a month, although you may be able to speed things up if we determine you could take some mature media from one of your other filter.
Meanwhile, you need to be the "manual filter" for your fish by frequently changing significant amounts of water. Your goal will be to figure out a water change schedule and percentage based on the feedback of ammonia and nitrite readings from your API kit. You never want either toxin to go over 0.25ppm, using the water changes to hold it between zero and 0.25ppm.
~~waterdrop~~