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Is this normal? I fed my fish some bloodworms and I noticed that when one cory swallowed one, it poked right out of his left gill, it then went back in, back out again and then eventually started popping out of his right gill, unless im mistaken.. Does this normally happen? That was an incredibly strange night..and the day after when i decided to do a water change, 3 pencil fish suddenly disappeared from my tank and I swear that I didnt syphon them up...Nice.

Since there is a direct connection between the mouth and the gill openings, this frequently happens and is nothing to worry about.

3 pencil fish suddenly disappeared from my tank and I swear that I didnt syphon them up

What other fish are you keeping in same tank as those missing pencil's?

4 corys (now 6) and 2 dwarf cockatoo cichlids, although the pencil fish are a similar size to the larger male cichlid and they get along fine. I've only seen aggression from the pencil fish themselves. All I can assume is that they went up the syphon without me realising, or they decided to jump out of the hood while open, although I cant find any trace of them on the floor/in the filter media. The tank is 33 US gallons.

I purchased 2 more Corys yesterday from a pet shop, one is fine and happily eating away with my other corys, although one seemed to have swimbladder problems a few hours after being settled in, only being able to swim half way to the bottom of the tank. It soon just gave up and stopped moving, floating at the top of the tank..this morning it is wedged on a piece of java fern under the water and isnt moving, although its gills are still operating. Has its body just spazzed out? It looks a bit deformed, but perhaps that's just because he's pretty skinny...what are the chances of a fish surviving after it goes like this? All my other fish seem perfectly healthy, even my shy bandits are out and about today.

I dont think these results are accurate, as its hard to read the differences in colour on the strips i used:

Nitrate: 50 (a bit lower?)
Nitrite: 0
GH: 16d
KH 5d
PH: 7.8

Tap water:

Nitrate: 30
Nitrite: 0
GH: 16d
KH: 5d
PH: 8.2

Ive just come back for lunch and he is looking better...He was held under a different piece of java fern this time around (as if so he wouldnt float away) and I spent about 2 minutes wondering whether he was dead or not..he then came off the leaf and swam in a circle, without floating to the top of the tank. Hopefully he is getting better! I've turned the filter flow down so he wont get smashed around too much.

I dont think its anything to worry about, I see my corys pumping sand through their gills while they look for food.

Nitrate: 50 (a bit lower?)
Nitrite: 0
GH: 16d
KH 5d
PH: 7.8

Tap water:

Nitrate: 30
Nitrite: 0
GH: 16d
KH: 5d
PH: 8.2

The Ph is too high, and it may be that your pencils died as a result and the other tank mates quickly made a meal of them.
Your other tank mates will benifit by lowering your Ph to 7 or slightly below. There are many ways to achieve this lowering of Ph.
For example: -
1. Adding peat to your filter
2. using chemical additives (available at your LFS)
3. Some aquarists use filtered and/or rain water when there tapwater is not conducive for keeping the fish of their choice.