Disappearing Danios!

[KrystaK]

I have a 30 gal tank (3g x 18 x 12) and I've bee trying to introduce a school of Zebra Danios to it for about a month now.
I have had a school of four for about two weeks now, they're all adults, three males and one female I think. I have tried to introduce other fish, but they have all been young, less than an inch. I've introduced four, and they have all disappeared. I expect they're dead, but I've never found their bodies!
And now I'm missing my single female, I have no idea where she has disappeared to!

Are Danios known jumpers? I keep my water level to about an inch below the lip of the tank and I could see my cat having a lovely snack if she found them on the floor (She's trained not to go fishing in the tank).

I have a good number of cherry shrimp in the tank, could they finish off a Danio within a few hours? Could that explain why I never find any? But that still doesn't explain why they died. My Pearl Gourami is rather territorial, but if they were in a large enough school she shouldn't have been able to stress out any one in particular to death.

Are Danio's sensitive to pH? Mine is high at 8.0, and if that's why they're dying I would feel terrible.

All my other readings are normal, my pH is the only off reading and I do weekly water changes of 30-40%, which is about 11 gallons.
My stock is:
1 Female Pearl Gourami
1 Male BNP
4 (or 3) Zebra Danios
6 Panda Corys
Lots of Cherry shrimp

 

[snazy]

What temperature do you keep the tank at?
Are you sure the cat is really trained? I wouldn't trust mine with an open tank and although he doesn't go near them normally, he is obsessed with fish food and if I leave anything unattended he goes for it behnd my back, that includes blood worms/brineshrimp and you name it?
It would be strange not to see at least the fish in near death mood if it was a sickness? It's possible that something is eating them too of course.

 

[KrystaK]

The temperature is about 76, give or take.
Yes I'm sure. I caught her with her paw in the tank once and I got her with so much lemon-water she was still damp an hour later. (We have a spray bottle of lemon water and we used it to train her to stay off some of the furniture - like bookshelves, the TV stand and the fish tank) She shows no interest in the tank what so ever now.

I saw one of the small fish, he appeared to have a swim bladder problem, he could only swim vertically and only for a very short time. So I posted in the emergency section, but the next morning he was gone

 

[snazy]

Sorry to hear that. Something is a miss. Keep in mind that just because they swim vertically doesn't mean they've got a swim bladder disease. It's actually not a disease but a side effect. Many fish swim vertically before they die. Hope you find out what it is.
From your initial post it seems your tank is new. You say other readings are normal but are ammonia and nitrIte 0? any other reading is not normal.
Also, the fish may have either been sick or a very good possibility is that you are not acclimating them correctly. If your ph is quite different than the one in the bag they came with, you need to do "drip acclimation". It's very important in cases like that.

 

[KrystaK]

My tank has been running for over a year. However about two months ago I replaced the filter pad and didn't put in enough old media, so I accidentally restarted the cycle. I paid for it with a good portion of my stock The readings have been stable since, with the ammonia between 0 and .25 (The tap water has some ammonia in it so when I do water changes I'm actually introducing more ammonia into the tank) Nitrate is not readable on my kit, but it's been that way since I started my tank last July. My tap water is terrible. I'm so glad we're moving.

To be honest the only reading that's ever right is Nitrite.
Ammonia is usually 0-.25, depending on how long after a water change I test the tank
Nitrate is unreadable, at the low end of the scale.
Nitrite is 0
pH is 8.0-8.2

As far as acclimating I drip acclimated for about 30 minutes before introducing them to the tank, which I thought was the right way to acclimate. If I have to wait longer I will.