Why are my GloFish fighting? What causes aggression and how to stop it?

This could be due to having a tank that is too small. 6 goldfish in one tank poses several potential issues. Ideally, 1 fish should have a 20 gallon tank with an additional 10 gallons provided per extra fish. This would mean they should be in at least a 70-75 gallon tank. The reason being goldfish produce a larger than average bio-load (wastes) that can quickly overwhelm a small tank and filter. Poor water quality can lead to stress and infection as well as behavioral issues. Depending on their sizes, the present tank could be overstocked leading to bullying and fin nipping. Check the water quality and chemistry. Use a home test kit or submit a water sample for testing. Most pet stores will do this for free or for a small fee. Aim for an absence (0 ppm) of ammonia and nitrites with a nitrate level of 20 ppm or less. Perform partial water changes, if indicated, to stabilize chemistry and maintain or improve water quality. Use only aged or conditioned water that is temperature matched to prevent further stress or shock. Goldfish are cold water fish and do best between 65 F and 72 F (18.3 - 22.2 C). Make sure the filter is rated for at least twice the volume of the tank. If possible, separate out the aggressive goldfish to a different tank to prevent further bullying and injury.

Thanks for your question about your fish, and congratulations on your new family members! I didn't see how recently the tank was set up, but there are a couple of things that could be going on. I would start by checking your water quality - any abnormalities in water quality can make fish have abnormal behaviors, and the water looks a bit murky in the photos you provided. There may be parameters that need to be addressed depending on what you find. If you do not have a water quality kit, they can be purchased at the pet store or an aquarium store, or you can take a sample of your water in for evaluation to these places as well. Some will charge a small fee for this service, just so you are prepared. Also, with recent tank set ups, it can be stressful for fish as they are in a new habitat. I would recommend keeping the light on for only a few hours per day to start, as the bright lights can be shocking for them (since fish don't have eyelids). Gradually you can increase the length of time that the lights are on so that you reach a cycle of 10-12 hours of light and 12-14 hours of darkness for them. I hope this gives you a place to start, and I wish you and your mom the best with the new pets!

Remove them or provide more cover (plants and hides) for the other fish to hide or escape. When fish hang near the top, it is usually due to low oxygen levels, low water temperature or improper water chemistry. It could be related to the different filter or the water change. Confirm proper water temp and use a test kit to check water chemistry. Ammonia and nitrites should be at 0 ppm (parts per million) and nitrates should be 40 ppm or less. Monitor chemistry closely until you know how the new filter operates. Make sure it is rated for the size tank you have to ensure adequate filtration. Correct imbalances using chemical treatment and additional partial water changes using aged/conditioned water.

Check the water chemistry and water temperature. If it is a new tank, it may not be properly cycled and could be undergoing and ammonia or nitrite spike. Either can drive fish to the surface or to one area of the tank. Submit a water sample for testing or use a liquid master test kit at home. Aim for 0 ppm ammonia and nitrites with nitrates at 40 ppm or less. Water temp should be in the mid 70s F. Perform a partial water change (30%) if necessary to stabilize chemistry and maintain water quality. Use only aged or conditioned water that is temperature matched o prevent stress or shock. Survey the area around the tank to rule out negative stimuli such as bright/direct light or loud noises/vibrations. These can also stress fish and cause them to move as far away as possible. Make sure the tank is large enough to support 8 guppies. Overcrowding can cause stress and aggression. Lastly, provide plenty of cover in the form of live or artificial plants. Guppies prefer cover and guppy fry (babies) need it or they will get eaten.

You can try to provide more hiding areas for your goldfish, and this may allow for it to escape from the pleco. If this doesn't work, you may need to add a barrier into the tank in order to separate the two of them. Barriers can be purchased at your local pet store, and come in a variety of shapes and sizes depending on the type/size of tank that you have. If you are able to gather information on the dimensions of your tank (gallons, length, width, and height) and take that information with you, it may be helpful for the pet store employees. If this is a recent change, you may also see if your pleco is thin - sometimes fish will become aggressive if they are hungry. You can get a general idea of the body condition of your fish by looking at their heads in comparison to their body size - if the head looks too large for the body, that can be an indicator that they are thin. Another way to tell is by looking at the belly - if it looks sunken, that is another indicator that the fish is thin. You may be able to take photos into your pet store to see if the fish are thin, or you can post another question with photos here and we can take a look as well. If you would like to discuss things further, I would be happy to do a consultation with you as well. I hope that gives you a place to start, and good luck!

Unfortunately, aggression between species of fish is not uncommon. I am not sure if this is a new finding, or if anything has changed in the dynamic in your school of fish (any new introductions or any deaths of fish), but this often can then change how the rest of the fish interact with one another. You can try to give the platy places to hide and escape from the guppy, or you can end up dividing the tank. If you do not want to add dividers, you can also separate the guppy into another tank as well. I hope that gives you a place to start, and good luck!

Danios are schooling fish. You need more than two. At least 5-6 would be optimal. Tetras are also schooling fish. Two groups of schooling fish requires a large tank, at least 20 gallons. The behavior is likely stress related due to the lack of an adequate group for them. Those issues aside, mollies and tetras have different water requirements. Mollies thrive in hard, alkaline water while Tetras prefer soft, acidic water.