How do high nitrates affects fish?
Poor appetite, insipid coloration, tumors, fin-rot and a generally lowered resistance to disease.
Can nitrates be introduced in any other way apart from in the main water supply?
Yes, some low-grade salt mixes have been found to contain nitrates, as have some algal fertilizers. It would be wise to test your chosen brand of salt mix after making it up with distilled, or other nitrate-free water. Algal fertilizers are nearly always based on sodium nitrate and they should also be tested. Low-grade carbon can also be source of nitrates, although most marine carbons are usually nitrate-free.

Can high nitrate levels indicate a wider problem?
Yes. High levels of nitrate are usually associated with deteriorating water conditions, and water changes should be increased to compensate.
Will changing half the water in the aquarium decrease nitrate levels by 50%?
A 50% water change will not reduce nitrate levels by 50% . Only continuous regular water changes will achieve a lasting reduction.
Why do some test kits show their results in different scales?
Some manufacturers have chosen to measure the nitrogen ion(NO3-N), while others measure total nitrate (NO3). To convert NO3-N to NO-N divide by 4.4 levels in this book are always quoted as total nitrate NO3. To avoid confusion, always check the scale on the test kit before purchase.