I think it is "slag" from metal ore processing. Depending where you are in the UK you should be able to find which metals were mined and refined locally. If it is from steel production it will be quite limy ("basic slag") and contain some phosphorus.even the melting bit seems to point to a igneous rock such as basalt.
I don't see why you need to be ashamed. Aren't people running around in a hysteria because they think they need more Iron in the plants diet? There is no reason whatsoever to ban Iron containing rocks from your tank. Iron is a commodity.Yup there's iron in it. I chipped a tiny amount off and attracted it to a magnet. Bit of a shame as I like the look of it. Anyway I think the other two will be fine. I may set about the larger one with a chisel, as its a real big lump.