It looks like it. I looked at the OP's location and assumed the friend was also in England so I looked at Tetra's UK site. I've just looked at Maidenhead Aquatics' website and that also says 5 ml in 10 litres (Maidenhead Aquatics is a UK wide chain of fish stores)Maybe its a different thing in the UK so you have to add a different amount???
Tetra AquaSafe
Tetra AquaSafe makes tap water safe for fish and reliably protects fish and plants against harmful substances contained in the water. The optimised vitamin B mix now reduces stress experienced by fish during water changes. 5ml treats 10L of water.
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