Tilapia: Is it hardy enough?

Tilapia are considered one of the fastest breeding fish. But it is considered hardy enough? I bought 8 tilapias (fingerlings) last week at a price of RM1 ($1 is equivalent to RM3.50 so do your own math) and by today all are dead. So i say to myself "What the hell is this?". The first one died because it jump out while transportation. Then 3 more died because of fin rot. Then 3 more died for unknown reason after I gave some medication and added some salt. And the last one died today. So this fish is not as hardy as it tought it be.