No, the water they used was blue/clear, not red.
The initial tank fill was red, you can see it in the other shots, they were shot before they lowered Mark into the tank.
Peter Suschitzky (the DP) decided the scene with Luke in the tank would have more impact If the tank was the main source of light in the scene. The did a test with a WWII searchlight firing at a mirror suspended above the tank. It flooded the room with red light, and they were worried it wouldn’t pick up well on film, and that it would make Luke look like he was being cooked. After the test and just before Luke was about to be lowered in, the big mirror shattered, sending huge, potentially deadly shards into the tank at speed. The thought on set was that had Mark been in the tank he might not have survived. They drained the tank, removed the glass and refilled it with chlorinated water. The lighting looked much better, less light was required and Luke was more visible, so they shot it with the tank ‘blue’.
The scenes shot before the accident have the original red liquid in there. There are photos of mark being lifted out of the tank, under normal lighting and the water is clear/bluish, and poor Mark looks like a drowned rat.
Also, On the print you can see the rainbow ribbon wire near Luke’s mouth, and the full colour spectrum is present. Had the water been even mildly red, those colours would not be there.