I gave this product 5 stars because it works, BUT it's not value for money. This is because this is not a concentrated version, the whole 100ml is only good to treat around 60l of water. This is obviously not enough after two 50% water changes on a 70l tank... You'll get much more value paying £1 or £2 more and buying Interpet Bioactive Tapsafe, that product is also around 120ml per container, but it treats a lot more water because it is more concentrated.
If you search for it, it is a blue bottle and the box is also blue with a photo of a real fish. I recommend that one as it will last ages. Unfortunately this Gold range is made for fish bowls, not aquariums. I believe that they should state this on the description but they don't, so you might make a mistake like I did and buy it thinking it's for goldfish and that it will treat lots and lots of water but it won't. It's for old fashioned bowls where fish shouldn't really be kept.
Please spend a little more and buy the right product, it will treat you hundreds of litres of water instead of a few tens of litres like this particular one. It's the same brand, but different concentrations... I gave it five stars because it works like I stated previously, it's just a matter of getting value for money in treating 50l versus hundreds of litres with the other product. I hope that helps.