Need Help With Cherry Barbs - Internal Parasites?

[Elisabeth83]

So after 2 months of treating my aquarium on and off for internal parsites my Cherry Barbs are still unwell. Basically this is what is happening - I had a group of Cherry Barbs as well as a Bristlenose Pleco and after noticing them going off of their food and getting thin and having white stringy poo I thought it was Internal Parasites and rang my LFS to see what treatments they had. I live in Australia and found out that they can only sell Praziquantel which only treats some Internal Parasites. Since my fish weren't displaying any red worms from their bottoms I went on the advice of the guy at the shop and bought the Praziquantel and treated the tank. At first I thought the fish were getting better but after doing 2 treatments total over a period of 2-3 weeks the sick ones weren't getting any better and I ended up losing about 3 Cherry Barbs and the Pleco. I decided to get my hands on some Metronidazole from the Vets and did 1 round of treatment (300mg in the water every day for 5 days and a 50% water change everyday) which really perked them up and I thought everything was good. They were buzzing around in a group and eating again and a couple had normal poo again. Last week I noticed 2 of the females going off of their food again and getting really thin. I started treatment again and am on to day 4 of 5 with 2 females now pretty much just hovering in the tank either at the top or bottom not doing much, not eating and looking quite thin. I don't really know what else to do anymore? I thought I was experienced in fishkeeping but I can't figure this one out therefore I am finally coming on here to ask for help from others who may have experienced this before.
 
Thanks!
 
edit - sorry forgot to mention to begin with the barbs were flashing on the driftwood and sand (not ich or velvet) that's why one of the reasons I went with Praziquantel thinking Gill Flukes.