In article ,
jd wrote:
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.

Could be the plants ran out of something, like iron. Or it could
be your water changed chemistry. It does that out of the tap in
some places in the spring and fall.

If it were me Id change 80% of the water for two days then fertilize properly.

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