Dead glowlights

[minx]

Perhaps somebody can give me a few ideas. I found all 4 of my glowlight
tetra dead this morning. Well, 3 of them, the 4th is still missing in action
(I think he may have been eaten by one of my 2 Corys)

I'm at a bit of a loss as to why this happened. They were all fine lively &
feeding yesterday. I changed 25% of the water on Tuesday evening as normal.
I added some algae control stuff yesterday (but have used this regularly in
the past)

The only thing I can think of is the fact I changed half the Biomax in my
filter for the first time. However I've checked temperature, pH, ammonia &
nitrite & they're all normal. What could have gone wrong? They didn't show
any sign of disease or anything.

I'm a little suspicious of the Corys ... they've grown a lot recently and
seem perfectly capable of attacking the tetra (although I've never seen them
show any interest). I've only got 2 - they have always been happy as a pair
until a month or two ago when they seemed to get a bit nervous & edgy. I was
planning on getting them some buddies this week but I'm a bit wary given my
gory find this morning.

So, now I'm left with two Corys. I've been really pleased with them, I'd
like a couple more to make a proper shoal.

Can anybody suggest a suitable compliment for the Corys? I've had my 10
gallon tank for just over 6 months but would still consider myself a bit of
a beginner.

[fishfreek]

However I've checked temperature, pH, ammonia &
nitrite & they're all normal

Do you recall your acutal levels. If so what where the acutal levels.

My best guess is that you had a micro ammonia spike since you did say you changed out 1/2 of your bio filter materal.

It seems a big of a big change for a 10 gal tank that would have had 6 fish in it prior to the change.

[minx]

I can't remember the values offhand, they were the lowest band, pH about 7.5.

I usually change about 25% every week, but I'm coming to the conclusion this might be a bit much. The only thing I did different from usual is change the Biomax in the filter .... perhaps this was just a bit too much.

Anyhow, now I've got to slowly rebuild my tank. I've just got a couple of Corys to keep my pair company. Next plan is to get an Otos or two to clean up the algae.

[fishfreek]

Any detectible level of ammoinia is to high for an established tank. If your constantly having ammonia levels being detectiable when you test then your either overstocked, feeding to much, or have inadiquate filtration for your needs.

I would advise just one Otos given this is a 10 gal tank once the bulk of the algae is gone if you have to many algae eaters then you either have to suplement their diet or one or both could starve.

[CRaZeeeBiTcHisH]

nitrite should be unmeasurable in an established tank as well as ammonia....what about nitrates? (grasping at straws here )just sounds very unusual for them to be fine and then dead....how long had you had the glowlights? i am guessing that the minispike from the filter change may have done it, but if your tank is well established, I wouldn't think it would spike to high or too long, it should have been able to catch up...you don't scrub your gravel and ornaments, plants etc. every week when you change water do you? best wishes for your fishes