Thanks for the reply and info - I gave up on the fish for awhile.  The aquarium I purchased them from wanted me to complete the sterazin course (10 days) before doing anything else and the vet who rang me back said levamisole was very expensive and that ivermectin was the only med I needed - cost $25 for the med and $20 to talk to him for 5 mins.  Very easy to overdose on ivermectin so I didn't purchase.  

So last week I finished the Sterazin course and one fish lost it's worms but the other didn't.  Have been really busy but have just noticed a couple of fish are now not very happy.  Deliberating over whether to take the contents of my tank back to the aquarium shop.  All have been purchased from her in the last few months so she said she would take them back which is very good - but once I start messing with levamisole or ivermectin she doesn't guarantee what might happen and she also insisted that sterazin worked on their very expensive discus(?) fish.

I rang Sykes Vet international and it is $21 to deliver the levamisole to my door - she said the Big L is 200 g/L ie 200 mg/ml NOT 27 g/L you mention inyour post.  

I worked out the conc in the tank needs to be 15 to 30 mg/L so this is 7.5 - 15 ml for a 100L tank if it is 200 mg/ml conc but is 50 to 100 ml for a 100 L tank if is is 27 mg/ml like you mention.  (Don't know how all you guys in the US manage your math when everything is constantly confused by gallons)

Can you let me know what you worked out for your tank?

Thanks also for your thoughts on rain water - I read something somewhere too - but I can never find it again!  We boil the water for drinking so I don't fancy boiling for water changes too...  We have to buy a filter/distiller or something so I will check out reverse ozmosis too.