Hi Gaina...Lots of confusion generally surrounding Milton Fluid.
I set up Milton BabyCare in the UK and can promise you that although Milton Fluid smells like bleach, it is NOT bleach and is completely safe for cleaning fish tanks and associated aquatic equipment.
Milton Fluid is made of an aqueous solution of sodium hypochlorite and 16.5% sodium chloride. The Milton Fluid in the UK has a strength of 2% sodium hypochlorite.
What makes Milton Fluid different from bleach is the purification process during manufacture which ensures complete removal of all heavy metal ions which would normally act as a catalyst to chemically break down hypochlorites causing instability. With the heavy ions removed, Milton Fluid decomposes into water and a small amount of sodium chloride (salt).
I use Milton fluid all the time...It is really effective for killing stubborn bacteria and is especially effective when rinsing out old tanks.