Hi and welcome to the joys of pets for Xmas
The best thing to do is reduce feeding to 2-3 times per week, and do big regular water changes.
The less food going into the tank, the less ammonia that is produced and the lower the ammonia levels remain. Don't worry about the fish starving during the time because unlike mammals and birds that use most of the food they eat to keep warm, most fish take their body temperature from the surrounding water and any food they eat is used for growth and movement. This allows fish to go for weeks or even months without food and not die.
After the tank has cycled (in about 4-6 weeks), you can feed the fish every day.
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Do a 75% water change and gravel clean the substrate 4-8 hours after feeding the fish. This will remove uneaten food and any ammonia that was produced by the fish and the food.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Monitor the ammonia and nitrite levels and do a 75% water change if there is an ammonia or nitrite reading above 0ppm.
When the filter has finished cycling and you no longer get an ammonia or nitrite reading and they remain on 0ppm at all times, you can do a water change and gravel clean the substrate once a week.
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If you have a light unit above the aquarium, you can add some live plants and they will use some of the ammonia and help keep the water safe for the fishes. If you don't have a light unit above the tank, don't bother with live plants because they won't survive without some form of lighting.
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API QuickStart is a filter bacteria supplement and helps speed up the cycling process. I recommend using a double dose every day for a week, and then pouring the remaining contents into the tank. Try to add the bacteria near the filter intake so it gets drawn into the filter where it belongs.
Add the bacteria supplement after a water change so you don't remove it when you do a water change.
You can't overdose with the liquid bacterial supplements so don't worry about putting too much in. It is naturally occurring bacteria that normally live in an aquarium.
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When you test the tank water during the cycling phase, do not test for nitrates. Nitrate test kits read nitrite as nitrate, and give you a false reading.
During the first week, just test for ammonia. In the second week you can start testing for nitrite as well.
When the ammonia has gone up and come back down to 0ppm, and the nitrite has gone up and come back down to 0ppm, the nitrate level should start to go up. When this happens, the tank will be considered cycled.
The cycling process normally takes between 4 & 6 weeks but can take longer in some tanks, and it can take less time if you add the filter bacteria supplements.
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If you post pictures of the fish we should be able to identify them for you.
If you have a small species of suckermouth catfish (Pleco) you can probably keep it, but if it's a common Plecostomus, they grow to 18-24 inches long and should not be kept in aquariums.
All suckermouth catfish need algae and driftwood in their diet. You can buy small pieces of driftwood from most pet shops. Just hose it down and put it in the tank. Driftwood will usually release tannins (yellow/ brown stain) into the water and this is perfectly normal and nothing to worry about. However, if the tannins get too dark, it makes it difficult to see the fish. The easiest way to remove tannins from an aquarium is with big water changes. After a period of time, the driftwood stops releasing tannins and the water will no longer turn yellow or brown.
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Try not to worry too much. Keep the feeding down and water changes up and the remaining fish should be ok. If you have any problems, pop on here and post some pictures of the fish sick fish and we will try to help you.
The following link has information about what to do if your fish get sick. It's long and boring but worth a read when you have some spare time.
If your fish ever looks sick or unwell, then the following steps might help. Test the Water and Clean the Tank. Test the water quality for ammonia, nitrite, nitrate and pH and write the results down in numbers. Check it for general hardness (GH) and carbonate hardness (KH) too if you can, but...
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