Massive white patches are killing my fish! (with pic)

[hitty]

Hi guys and girls...

I've got a 55 gallon tank pretty much stocked to the max with a whole bunch of various fish (mainly tetras)

I haven't had a fish die on me for almost 3 months... untill.. 4 days ago when i woke up to find my only molly had a massive sore on its caudal peduncle... it was a white fuzzyness (not the cotten wool fungus) with a hint of pink... the guy didn't look well, and i decided i'd put him down when i got back from school...
when i got back it was too late.. i didn't think too much of it, all the other fish looked fine... the next day... I lost a kerri tetra, a koolhi loach and a rummy...

in goes some antibiotics (tetracycline hydrochloride) the water goes a disgusting yellow colour and some of my plants start losing leaves... the fish breath a little heavier but no signs of this disease...

2 days later (today) i wake up intending to do a massive water change to get rid of the antibiotics... when i notice a kerri with the same kind of wound in the same place, its back is bent too.. so i took it out... hes waiting on death row and a otherwise fat healthy and colourfull rummy has almost all of its caudal peduncal this off white colour.. i'm guessing by tonight it is a goner too

so what i'm asking is, what is this stuff?? how can i stop it? and how did it get into my tank considering i haven't added any fish or plants into it for two weeks (2 upside down cats and 6 more glowlights)

bizzare right? some other information, i do 30% water changes every two weeks, temp is at 25c... and the most vicious fish i have in there are nothing more deadly than a few gouramis...

thank you all in advance

 

[RoK]

Well, actually sometimes gouramis can be deadly, depends on the gourami's temperament. I looked in your profile and am I right you have a male betta in there? It's possible some of your fish are biting them or ramming into them when you're not looking.
If not then it probably is a disease, although it's hard to tell what it could be with the way you described it. Also, what are your water parameters? (pH, nitrite, nitrate, ammonia)

 

[hitty]

ahhhh my profile is outdated... i'll fix it up... or next week when i wont have to type so much the fighting fish was moved to his own seperate tank along time ago..

my gouramis are pretty good usualy.. they'll chase eachother around the tank but they never annoy any other fish...

as for my parameters.. all i can tell you is that the ph is 7.0 and seeing i just finished a 50% water the others should be low right?

 

[hitty]

ok, the latest rummy just died...

theres a hi res pic of it here.. although it didnt come out too clear.. you can c how the back half of the body is a different shade.. and the tail is kind of pink..

ignor the clamped fins, yesterday this was the biggest and healthiest rummy i had seen



heres the link...

i've got 10 more rummys... (crosses fingers)

 

[Menagerie]

all i can tell you is that the ph is 7.0 and seeing i just finished a 50% water the others should be low right

Best to get the water tested.