Hi, I've had a look round the forum and couldn't find the answer to my question, but if it is a re-post I apologise! My son has a ruby shark in his tank, which has been happy enough until a week or so ago, when we noticed it's changed colour from silver to black, and it's eyes are 'bugging' out. It's always seemed ok in the tank, and claimed a hollow skull as its home as soon as it was put in. We haven't changed the food or the times between cleaning or introduced any new fish for at least 2 months. It's my sons fave fish, so I'd just like some advice on whether this is normal or not. Thank-you!

Ruby sharks can be grey or black. It depends on their mood. However, the bulging eyes could be caused by a bacterial infection. You might need to treat the tank with a broad spectrum anti bacterial medication to treat the fish. "Waterlife Protozin" or "Wardley's Promethyasul" should do the job.

To work out the volume of water in the tank
measure Length x Width x Height in cm
divide by 1000
equals volume in litres

When measuring the height, measure from the top of the gravel to the top of the water level. If you have big rocks or driftwood in the tank, remove them before measuring the height.

Before you treat the tank do a 50% water change and complete gravel clean. This will reduce the gunk in the tank and allow the medication to work more effectively. It will also lower the pathogen count in the water and mean there are less nasties around to infect the fish.

Remove carbon from the filter before treating otherwise it will absorb the medication out of the water.

Thanks for the swift reply! Unfortunately, I left it too long before asking the question cos the fish has died. My son wants to go out and buy another ruby shark, which we could do tomorrow, but would you recommend treating the water as you describe before we do that or not? He's got 8 platys (I think that's how you spell it!) and 3 fan-tail guppys and they are all doing fine. Your help is very much appreciated. Cheers!

Don’t get any new fish for at least a couple of weeks. Let the tank settle down and if no more fish get sick then get another shark.

If any more fish do get sick, have buggy eyes or get red or white patches on them, then treat them. Otherwise just wait a few weeks and then get another shark.

It’s always a good idea to do a partial water change and gravel clean after a fish dies in the tank. This reduces the number of disease organisms in the water and can help reduce the risk of further problems.

Make sure any new water is free of chlorine and has a similar temperature to the tank water.