Stupid question - bad tank?

I have this little 5 gallon and had prepared it for set up for thepygmy cories, I was going to clone the filter (ready and set up in another tank to transfer with introduction of fish) and testing this morning, everything including water params were fine...when I got home, ready to introduce them in but the three lamp fry i had in there were glued to the top and alarm bells were triggering. Upon testing the water was extremely high in nitrites! Have i misunderstood something or this is unusal in less than 12 hours? I ended up setting up a seprate tank on short notice and putting everything in there with the fully cycled filter, luckily the fry I have had survived everthing (and probably would a nuclear bomb!).

The reason I also ask can you get disfunctional tanks is, this was the tank that had the fry die in and a day later (hadnt got rid of the water but had the fry bodies) there were extremely high nitrites. The bottom of this tank has a webbing of metal in the glass/plastic. Can something be effecting the tank and should I not use this anymore? Would it be useful to leave it with fresh water and test daily?

Sorry to always come up with ridiculous questions!

Thats very unusual for nitrItes to get high in such a short space of time??
Only thing you can do is a big water change to get the nitrItes down:)

The webbing you mention will be embedded in the glass, so will not affect the water in any way.

Ah that is good to know, we are not using this tank currently but will investigate it further (the only real time ever had nitrites or ammonia was my first tank with the dwarf puffs and the last stage of a mini cycle so find this baffling!) Thanks Van (you sort me out! LOL)