Confusion With Api And Nutrafin Liquid Tests

[Kaidonni]

So I decided to do a last-minute nitrite test with my Nutrafin liquid test kit last night and got an ever-so-slight tinge to the results (we're not even talking 0.05ppm, possibly lower than 0.01, and the test actually starts at 0.1). Panicked, put some replacement water to use today, and re-tested this morning...if there is a tinge there, it's even fainter, or I'm just seeing things. What might cause this? I clean the filter every Saturday/Sunday (this time around I decided Saturday) and Wednesday, along with a 10.5 litre water change and gravel vacuum (45-50 litre tank) - could I be disturbing the bacteria in the filter? I did change a quarter of the foam media the other week (Fluval 2-plus), by which I mean I have adopted an approach where when I replace the long-lasting foam pads, I replace half of one (and there are two) so as to avoid mini-cycles if possible. Where did these nitrites come from and then where did they go (well, I know it's all to do with the nitrogen cycle, but if I had a serious problem with nitrites, I wouldn't expect them to appear and disappear like that)?
 
I was going to do an emergency change today (well, I was prepared to), but with such miniscule levels that then disappear, I am concerned that if I do anything too soon I'll disturb things again.
 
I am in the process of rehoming a fish unsuitable for this tank size, a Clown Loach, and have 5xWhite Cloud Mountain Minnows, feeding them every other day. I keep on top of water changes, but am I doing anything wrong by cleaning the filter so often?
 
Also, as I have both Nutrafin and API liquid tests, what do I go with for nitrates? API measures total nitrates, so the results are 4.4 times greater than with Nutrafin (which measures nitrate-nitrogen). Either I'm in the range of 5-10 or 10-20 with Nutrafin, or over 40ppm with API.

 

[Kaidonni]

Have to amend part of the above post - it either looks 5-10/10-20, closer to 10, or 20-50ppm - possibly somewhere in the middle, ~30-40ppm - with Nutrafin. It depends. I usually conduct two tests, shaking the bottles hard, and it's the second that comes out darker than the first.
 
It's possible I need a new filter housing as a number of times I've had to take it apart, clean the rota, and put it back together when it wouldn't start back up. Perhaps the flow in the tank is reducing, causing problems (not as regards the nitrate test, but certainly on the whole it can't be that good to have to mess around like that). I do clean the rota and filter housing of gunk regularly now, but for it not to start sometimes and require further fiddling...

 

[Essjay]

When I had Fluval internals I found they didn't start after being turned off. I found the only way to start them again was to lift the pump off the filter and with the outlet pointing downwards, turn the power on and flick the impeller. That got it going then it was just a case of putting the pump back on the filter without soaking myself.

 

[ellena]

Why do you change the foam? Just rinsing in tank water should keep it working fine, as long as water is flowing through the filter ok, there's no reason to change the foam

 

[Kaidonni]

I changed the foam because it was in a rather deteriorated state - this foam had been in since 2009. The polyester pads that go in the middle only last about a week (they used to last longer...), but the foam pads last a lot longer - just it was high time I replaced some. I did the same back in November 2012 with the other half, but didn't account for mini-cycles, which led to problems. This year I've completely replaced a foam pad by folding the old one over and putting in half of a new one, waiting about half a year, and then removing the remains of the old one and putting in the other half of the new one.

 

[ellena]

Got you, I thought you were saying it was a regular thing

 

[Kaidonni]

Anyone able to answer my main queries on total nitrate versus nitrate-nitrogen?