Sick Fish What Can I Do

[sillysallyuk]

hi, i hope someone can help as i just don't know what else i can do. i have had tropical fish for 6 years with no problems at all. no sick fish, algee etc. then about 6mth ago i got some guppies, and within a couple of days one died and after a week all 6 had gone but i guess one of them was sick when i brought them. and within a day i noticed a mollie and a catfish had something like finrot. i put in fin rot meds and after losing half my fish over 2 weeks it cleared up. then a couple of weeks later it was back again to got more meds and lost more fish. angain it cleared up and all seemed well then the last few fish died other than my main 5yr sail fin plec. he had had no problems but really wasn't good so in a last attempt i put him in my plant growing tank with meds. after a couple of weeks he was back to normal. i stripped down the other tank, got rid of all water, stones, wood and everything. cleaned it all down. and scrubbed the filter etc. brought new sponges etc. put new stuff in with more treatment to make sure and filter start and had it running without fish. after 3-4 weeks i put my place back in. and all seemed well. after about 8 weeks we got 6 mollies. after 3 weeks they got sick and for the last few months i just seem to have to keep treating them. and my water over night went whitish. i heard that the treatment might have the biobrick filter thingys so i got more and filter start etc and water changed every week instead of every 2 weeks and it has improved a little. i have just finished one treatment that has sorted out some fish and now my plec seems sick again and my catfish seems to have finrot. it feels like im just throwing money away and it it wasnt for my plec which is like puppy to me i would have got rid of them all now.

im very desperate and would be greatful for any help.

thanks
sally