My tank has not had a water change in roughly a month or so and these are the vitals:

Size: 60 Imperial Gallons or 70 US Gallons or 260 Liters
Inhabitants: 1 growing oscar: + - 16cm or 6.4inches
2 Silver Dollars 1x 9cm/3.6inches and 1x 11cm/4.4inches
1 pleco: + - 12cm or 4.8inches

NO3: + - 50 mg/l
NO2: 0 mg/l
GH: <3d - >6d
KH: 0d - 3d
pH: + - 6

Tested water from my tap:

NO3: 0mg/l
NO2: 0mg/l
GH: just above <3d
KH: 0d - 3d
pH: 6.4 - 6.8

Here's the problem:

I watch my tank carefully and this morning noted that my largest silver dollar is struggling to stay at the bottom of the tank, the moment it stops swimming it floats to the top. Yesterday all was well with all the inhabitants of my tank. As far as I know the problem is rooted in the swim bladder. Can someone please tell me what the cause may be and what I can do to save my baby. I usually do monthly to bi-monthly water changes of 50 - 75% at a time and vacuum the gravel at the same time. I feed roughly every 2 days.

The reason for not having done a water change in over a month is that I ws in an accident on 27/12/2008 and cant do it at the momant so I have to wait for someone to do it for me. I will however arrange for more frequent changes if it will help?

Please help.

Thank you.
Wilde2angel.

Its very likely to be swim bladder infection. There seems to be a number of recent cases. If you look below your post a few others have the same problem. the infection can make the fish float or sink loose balance etc. Its poor water thats the cause, this is why we stress a wkly water change. large fish make a mess and screw up your water fast. treatment is turn temp up to 27c and a swimbladder infection treatment plus aquarium salt. "interpet" works ok and dont mess up your filter. also remove your carbon if used. it far cheaper to make those water changes than treat a tank of sick fish. without treatment the fish will stress and very likely die. good luck