Dear Scientist (???) Rojak,
You have reading problem, to say the least! Did I say fishes don't have cancer or never have cancer? All living things, including plants can get cancer. But my point is that it is very, very rare, so rare that we don't even have heard about it......So, yes, fishes do and can get cancer but because fishes in their natural habitat (i.e. the oceans) are being fed on perfect marine diets which have all the necessary elements (or nutrients) for their biological growth, they very, very rarely have cancers. Many of the cancers are caused because of poor diets and given the current state of food production which many of them contained preservatives and artificial colouring, etc, we find that cancers are now so common among people. Every one other of our friends who passed away seemed to be dying of cancer....this is not an over-statement....it's just a medical fact!
Yes....current technology cannot extract minerals from sea-water....two main reasons (among others): First the minerals are in ionised form (or liquid form) and second the concentration is not big enough for even the latest state-of-the-art membrane to extract its minerals. At best, a small amount is extracted purely for research purpose. In the oil crisis back in the 1970s, there were intense discussions to extract even petroleumn from sea-water but the idea was abandoned because it is simply commerically not viaible. It is still a dream for technologists to come out with a technology like creating an ultra-efficient membrane to extract minerals from sea-water....it is going to be a dream for a long time to come.....
Frankly, if there is an efficient and cost-effectively technology to harvest minerals from sea-water, can you imagine the state of the oceans today!...by God's grace and mercy, He has created this great treasure and prevent evil people from destroying it.....if and only if man can extract minerals from salt-water, cost-effectively.....all the oceans of the earth will now be devoided for marine life....just look at the rivers and the lakes today and we can see that the oceans will become no better than these rivers and lakes which provide the drinking water to all life....if and only if man can extract minerals from sea-water effectively and efficient, all the oceans would now have become dumping grounds for industrial wastes........