"jd" wrote in message
. ..
the tank is fairly heavily stocked with both plants and fish. Its a
community tank - guppies, swordatils, zebra danios, tiger barbs, croy
cats, an algae eater, and a fairly dense plant population - java ferns,
some assorted swords, and some random talling plants (LFS can't tell them
apart, and I don't bother)

Lightly fed on standard flake food twice daily (the food gets eaten up
within a couple of minutes, so I pretty sure I'm not overfeeding).

I do a 50% - 60% water change every week. Well water, run through an ion
exchangeer to remove excess iron.

Whats mystifying is that this tanks has been set up for a good while, and
has been stable. I haven't changed anything, but the nitrates suddenly
spiked, so something in the tank obviously changed.

--JD

Dead fish somewhere ?
Crud stuck in the filter ,plant material etc, ?
(check the hoses too)
Gravel needs hoovered ?
I'm out of ideas after that, hope you solve it.

P