Mystery Fish Deaths, Loosing Entire Stock

Hope somebody can shed some light on my problem, its a long story but I need to include all details to see where im going wrong.
 
Tank stats:
240 ltr
fluval external
hydor inline heater
inline uv steriliser
ammonia 0
nitrite 0
nitrate at current 20ppm
pH 7.0
26oC
2 airstones
general black gravel
bogwood
a few small live plants (unsure of species)
 
stock before problem:
2 plecos (one small, one medium)
10 neon tetra
8 serpae tetra
3 mollies
2 angelfish
2 clown loaches
2 asassin snails
4 small cory
 
food, bloodworm, tetra crisps, catfish pellets, algae wafers
 
I will start from the beginning. In june, everything fine and happy. weekly 25% wc with dechlorinator, nitrate never went above 60ppm.
 
Mid june I moved house, we did the move as quick as possible to avoid stress, I also took about 50ltrs of the old water, and also what was in the fish carriers. The tank was empty for less than an hour, filter back up and running within an hour and a half. Added some aquarium salt to help with stress, again everything ok.
 
About a month ago 2 serpae's died. No signs of infection or bloating. I tested the water and all was good but did a 50% change anyway. A week or so later another serpae and a cory died, again no signs of anything wrong with them but I did notice some white fungus around 2 of the serpae's mouths so I did another 50% change, removed the carbon from the filter and dosed with interpet no. 8, anti fungus and fin rot and turned off the uv.
 
Everything seemed happy and I did another 50% change a week later and dosed again. 2 days afterwards I got home and one pleco, 2 neons and another serpae was dead.I also noticed the filter was running slow (which was cleaned out 2 days before). I stripped the tank, cleaned all the pipework, gravel and glass. The filter had like a clear congealed paste all over the sponges, I also found lots in the pipework.
 
I got home from work Friday last week and noticed my male angel was not eating and his belly was slightly swollen, 2 hours later he was dead. Thinking it may be an internal bac infection I changed another 50% and dosed with interpet no. 9, anti internal bacteria. I redosed 4 days later as per instruction.
 
I just got home now and 2 more neons and a molly is dead. Also yesterday I noticed the female angel has an ulcer above her eye, which today is significantly bigger.
 
Has anyone any ideas because im stumped!!
 

Hmm, not too sure. but a couple of questions first.
 

kad191 said:

 
Everything seemed happy and I did another 50% change a week later and dosed again. 2 days afterwards I got home and one pleco, 2 neons and another serpae was dead.I also noticed the filter was running slow (which was cleaned out 2 days before). I stripped the tank, cleaned all the pipework, gravel and glass. The filter had like a clear congealed paste all over the sponges, I also found lots in the pipework.
 

Click to expand...

The filter, you say you cleaned this out 2 days before.
 
How did you clean the filter?
Did you clean the filter media? i.e in tap water or anything like that?
 
 
 

kad191 said:

 
The filter had like a clear congealed paste all over the sponges, I also found lots in the pipework.
 

Click to expand...

 
This sounds like either, the bacteria was trying to re-colonise or establish itself again.
Or could be a bacterial film which is typically normal for a new tank set up or begining of a cycle process.
 
Am not too sure if a cycle was going on but according to your tests, looks ok to me.

Thanks for your quick reply.
The tank has been cycled since feb, steady readings since then.
 
I clean the sponges in tank water and I never clean the media, it never gets dirty as the sponges pre clean the water.
It was weired, almost like a fine Vaseline. I was wondering if the anti fungus medication had solidified in the filter. There has been no sign since.

Yeah, does sound kinda strange, but your fish were dying BEFORE you added the medication so its possible your medication did not do very much apart from possibly solidifying the fungus bacteria. 
 
You moved house in June, is your new home tap water significantly different from your previous house? gH & gH as well as the usual tests for nitrite, ammonia, nitrate and pH

The pH, is the same, I have only just acquired the water hardness test strips and im going to the area I moved from tomorrow so I will get a water sample and post results. Its only 8 miles away so I cant see there being a mega difference. Would a big difference cause a problem like this, across all the fish species?? And what kind of difference would you expect to cause trouble?
 
I did have a thought that the stress of moving house could have knocked their immune systems and they have basically caught every disease going.
 
Regards the ulcer on the angelfish, what do you recommend the course of action to be?

 
Pic of ulcer