For those concerned about quality and integrity of supplements, I would suggest a subscription to ConsumerLab. I've used them for many years to identify and select supplements (along with foods and herbs) I was interested in. They do lab analysis of many different brands. I have been very surprised at the results of supplements from well known brands either having too much of a stated nutrient or not containing what the label claims or having high levels of cadmium and/or lead. In addition, they are up on all of the latest research on food, herbs and supplements and specify what those nutrients do in your body as well as cautions and concerns to consider. They are constantly updating their info with the latest available information. It is an excellent site and reputable source.
Be careful about buying supplements on Amazon if it is not handled directly through Amazon. There have been supplements containing content other than what is advertised and some laced with Viagra. The supplement brand NOW in particular has been counterfeited on Amazon along with others I'm sure. There is a small difference in the legitimate label vs the label of the counterfeited brand that enables you to tell.
I think Martin's statement re algae may have come from Consumer Lab's analysis several years ago as it was determined that AlgaeCal had a high amount of lead at that time. In years since, levels have come down some. Though our oceans are certainly polluted, we still eat fish and smaller fish have less heavy metals than larger fish and are still considered very healthy to consume so I wouldn't discount all from the ocean.