7 dead fish in 3 days

[spinningtop]

I don't know what's wrong with my fish.I've lost 7 in 3 days.They seem to be fine,then they go up to the surface and stay there for a while and then just die.I've lost 3 Guppies,2 Zebra Danios,1 Neon Tetra and 1 oto which I think was pregnant as it had a very huge belly.There's nothing to see on any of them.Anybody have an idea on what's causing it before I lose the whole tank?
My ammonia and PH are fine,i tested them today.
One of my Guppies also has tail rot,could that be affecting the other fish?I've quarantined him but I'll be surprised if he lives through the night as his tail's going really quickly.It's just like a skeleton.

 

[Purple]

get an airstone in there and see if that works

 

[spinningtop]

I've got 2 airstones in already plus my filter and my intake pipe.

 

[FroggyFox]

Sorry to hear about your losses

um...how about some more tank stats? You said there isn't any ammonia and the pH is stable (right?) but do you have nitrites? How long has the tank been set up, and how big is it?

Has your temperature been fluxuating at all? When was the last time you did a water change?

Oto's get huge bellies from eating...probably wasn't pregnant. They look like they swallowed a pea sometimes

 

[Lotus]

Have you added anything new to the tank (including new decorations or using new water treatments)? How often have you been doing water changes, when was the last water change? Are there any other visible signs on the fish?

The tail rot is probably not related, although it could be a sign of something not right with the water.

 

[melissakate]

it really sucks when fish just die for no apparent reason...... i keep having my mollies die. anyways keep trying and learn from your mistakes and you'll do fine. *PEACE!*

 

[spinningtop]

been away all wewekend so just got your replies.I have a 20 gal tank set up since November 2nd.Nitrates and temp are fine.Did a water change a few days before all this started.Somebody said add salt so i did that Friday and all I've lost since is 1 oto which had a huge belly.No visible signs of any kind on fish.How come my other 2 oto's aren't overeating and is this common for them? If I notice them getting big again is there anything I can give them to make them smaller? I wondered why they weren't doing their normal cleaning,guess that explains it.
I'll do a partial water change again tomorrow and see if that helps,i also now have some conditioner to put in the tank cause we're on well water here so i was wondering about the metals that could be in it.