1. Glucosamine is not a wonder drug. It does not replace/repair your joint cartilage. It has only been shown to reduce PAIN associated with arthritis. It will also not slow down arthritis.
2. Glucosamine has no effect on Calcium in the body. Vitamin D does.
3. Apparently Glucosamine + chondroitin sulfate is the preparation you should take. Glucosamine by itself is useless. (Related to me by a scientist; not backed by good clinical trials)
4. Fish oil has been shown to have an anti-inflammation effect ie. could potentially stop progress of arthritis. It has shown to have good symptomatic effect as well. Useful dosages are usually between 3g-7g daily. A lot of people do not take a sufficient dose for fish oil to be effective.
5. Neither fish oil nor glucosamine will 'cure' arthritis. If you have a joint problem please see an orthopaedic surgeon. Do not attempt to self-medicate. Alot of joint problems are easily treated. Some common joint problems like torn menisci in the knees (v.common for soccer players) if left untreated will cause early arthritis. No amount of fish oil or glucosamine will help you in such cases.
I'm also been taking glucosamine for a while now, but I only take it when my knees hurt. Anyway, I notice not all glucosamine is effective. The one I found that works for me is Viartril-S. It in power form and comes in small packets. Mix it with water and drink it.