Tested the water again this morning, nitrites were a similar level to yesterday, not much increase over night.
I did a 40-50% water change, which went much better, and have tested again. Its a little difficult to tell with my test kit because it shows a orange colour for 0.3mg/l and a yellow colour for <0.3 mg/l. My test was somewhere in-between these colours. I have decided, for the time being, to assume that yellow would effectively mean 0, so I think my test shows about 1.5 mg/l, which sounds right based on a 50% change.
My ammonia test vial has a small residue around the 5ml mark (the line I fill up to). I rinse out the vial after every test, and have started drying it as well. Is this typical, or is it going to risk producing false positives?
I also have a few questions about my fish. These may be silly questions, but I have a bit of a pessimistic outlook and always think the worst
The fish swim up and down the glass in the corner of my tank every so often. They don't do it all the time, but the behaviour is more evident in some of the fish and has increased, I think, since my first water change. I have read various different thoughts on this, some saying its normal, some saying its stress or boredom.
I have also noticed that they seems to 'surf' the current from the outlet of the filter. Again, I feel like it has increased since the change, but this may just be me noticing it more. I have seen different opinions on this as well, some indicating its for fun, some saying they are trying to find high oxygen areas of the tank. My tank has various live plans in it, which are growing, so I'm inclined to think its not the latter.
Finally, some of them do some weird sideways/backwards swimming thing. Its hard to explain, but their dorsal fin pops up and they do it for a second or two and then carry on like normal. Its probably nothing.
I'm not sure if these are signs of stress, or that they are more comfortable and active than they were a few days ago. My water conditions are:
PH 7 ish (test not very accurate)
Temperature 25
Ammonia and nitrites as mentioned
Nitrates not high enough to worry about
Thank again for all of your replies Sue, they have been very helpful.