I've evidence from various sources that for most of the 19th century aldulteration of beer was rife.
AS happened in these cases:
"BEER DILUTION.—At the Mansion House Police-court, on the 14th ult., George Frederick Davis, landlord of the “Coach and Horses Tavern,” Whitefriars-street, was fined £5 and £1 costs for diluting beer to the extent of 3.5 gallons of water in the 36 gallons. Henry Hooper, landlord of the “White Hart Tavern," Long-land, Smithfield, was fined, at the same court £5 and £3 10s. costs for a similar offence—At Bow-street Police court, on the 24th ult., Frank Bate, landlord of the “Enterprise" publichouse, Long-acre, appeared to a summons, issued at the instance of the Inland Revenue authorities, charging him with diluting beer. Mr. Alpe prosecuted; the defendant was represented by Mr. Crisp. According to the case for the prosecution, two Excise officers visited the defendant’s cellar on September 20 and took from a barrel a sample of beer. They told Mr. Soffey, the manager of the house, what they had done, and offered to leave a portion of the sample with him. He said, however, that they need not trouble to do so, as he knew the beer was all right. The sample taken was forwarded to Somerset House, and the analysts found that the beer had been diluted to the extent of 4.5 gallons per barrel. Mr. Crisp, for the defence, suggested that by some inadvertence diluted beer had been sent to the defendant’s house. The place was under the control of Mr. Soffey, the manager, who, when he was engaged, deposited £100, which he was to forfeit if he did anything likely to injure the house or prejudice the licence. Sir John Bridge said there could be no doubt that the beer had been diluted, and he was bound to convict the defendant. It was perfectly certain that the Legislature intended that a publichouse should be managed by the person to whom the licence was granted. The holder of the licence was responsible, and in this case he would be fined £25."To put those fines into context, a pint of Mild cost 2d per pint in the 1890s.
"The Brewers' Guardian 1892", 1892, page 363.
It looks like adding around 10% water was the standard practice.That's probably about as much as you can get away with before it becomde noticeable to drinkers.