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Check if your goldfish stays close to the surface. Watch your fish for unusual swimming patterns. Take a close look at your goldfish's fins and scale. Check if there's anything on your fish that wasn't there before. Pay close attention to your fish’s eyes. Make sure your goldfish is eating.

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If your goldfish spends a lot of time just below the water’s surface or seems to be gasping at any air bubbles, it may be a sign that your fish is having a difficult time breathing. This might be due to low levels of oxygen in the water, high levels of stress, or something else entirely. If your goldfish is swimming around with their mouth open, it may be another sign that they are having difficulties getting enough oxygen. Observe your fish for a few minutes at a time to see if they are swimming at an angle, upside down, or anything else that seems unusual. Goldfish will normally swim around in an upright position and stay nearer the bottom of the tank, so anything significantly different might be a sign that your goldfish is unwell. Similarly, make sure your goldfish is swimming around and interacting with things in the tank. If your goldfish is hardly moving and staying right at the bottom of the tank, it’s a sign that something may be seriously wrong with them. If you watch your fish often, you should have a rough idea of what they should look like. Watch your fish as they swim around, and see if you can get a close look at their scales and fins. Here are a few  things to look out for:  Look at your goldfish’s fins for any tearing or red veins that are bulging out. This is normally a sign of stress or could be an indicator that there is another fish in the tank that is bullying them. Make sure the fins are at their full length and not starting to shrink or deteriorate into a stump. If any part of your fish’s fins seems to be disappearing, it could a sign of fin rot. Black spots might mean that your goldfish has ammonia burn or some type of parasite. Make sure your fish isn’t bleeding or missing any scales. If you notice any small red marks on your fish or any spots in its skin without scales, something is hurting your fish. It could be another, more aggressive fish in the same tank, or it may be your fish rubbing against something to try and scratch a parasite. As your goldfish is swimming around, examine them for any new spots, bumps or textures on their skin and face. These could all be signs of parasites, bacterial infections, or other serious illnesses. Here are a few things you should keep an eye out for:  Look for a fine white fuzz, particularly around the mouth, or any other discoloration of the scales. This could be a sign of a bacterial infection that will need to be treated. See if you notice any small, white or black spots on your fish. White spots may indicate that your fish has ich, which is a common parasite found in goldfish. Check for bloated or raised scales. If you see any bloating or protuberance where you’ve not seen one before, it could mean that your goldfish is ill. However, it may also mean that they’re pregnant, or that you’ve just been overfeeding them. It can be very hard to get a good look at your fish’s eyes, but you should be able to inspect them with a bit of time. Look through the tank until your fish comes close enough to see their eyes. Unnatural bulging, a build-up of mucous or cloudy-looking eyes can all be signs that something is wrong.  Protruding eyes may be a sign of “pop eye”, which is usually caused by an infection. Cloudy eyes, or eyes with a cloudy film over them, can be a sign of some sort of vitamin deficiency. When you next feed your goldfish, keep an eye out to make sure all of them are swimming to try and get some food. Goldfish will eat anything they can, so goldfish that aren’t eating are likely very ill. If you notice that your fish isn’t eating, try sprinkling a little food so that it falls right in front of them. It may be that they can’t see the food or are slow to react to it, this is another sign that something is wrong.