If they were iron / steel, they would not be that toxic, but also they would be magnetic. Not sure how pure normal steel is though, its going to have some other metals in it I guess. If they are showing themselves to not be magnetic, they must be either stainless steel (unlikely), or brass (likely). If they are brass you want them out of your tank, because brass is about 50% copper, and 50% zinc. Both are toxic heavy metals, and brass will corrode in salt water IIRC fairly fast.
The tacks I've seen though were all iron / steel, so magnetic, although they were plated with brass, but probably not much. I just read that a typical brass plating thickness is 4 microns, so with a tack that has a head 0.3 inch in diameter, figure 1.5 square cm surface area, with 4 microns brass on it, is 6 miligrams brass. In a 200 litre tank 6 mg brass would be 30 parts per billion brass if it all corrodes, so 15 ppb copper and 15 ppb zinc. Randy tested his tank once at 13 ppb copper IIRC and his tank never showed any problem with that level.
I've lost many nuts and bolts in tight spots when working on various things, something that works for me if they are magnetic is to get a rod, or a bit of flexible pipe, tape a super powerful magnet to it, and slide it into the tight spot. The magnets you get in old hard disk drives are super powerful if you have any old disks you don't need. Old mag cleaner magnets probably are too. A mag cleaner rear surface probably won't work because the field strength is quite low on the non glass side of it. Or maybe turn pumps off and siphon it out?
