If you had the aquarium water temperature on 86F for 2 weeks, it is highly unlikely you have whitspot in the tank unless you have added new fish or plants since then.
The pleco does not appear to have whitespot. If you post pictures of the other fish I can check them but you can do it yourself too. Just look for small white dots on the fish's bodies and fins, they will be about the size of a grain of salt. Infected fish will usually rub on objects in the tank.
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Any filter that is running continuously in an aquarium for more than 2 months will develop colonies of beneficial filter bacteria that convert ammonia into nitrite, and nitrite into nitrate. This is the normal nitrite or filter cycle that we talk about in fish keeping. On average it takes about 4-6 weeks for this cycle to complete and during that time you get a build up of ammonia (caused by fish food, fish waste, rotting plants, dead fish, or anything else that breaks down in the water). After a couple of weeks you get colonies of good bacteria that appear in the tank and filter and they eat the ammonia and convert it into nitrite. The ammonia levels then come down and the nitrite levels start to go up.
When the nitrite levels start to go up, you get more good bacteria that start to grow in the filter and these convert nitrite into nitrate. A few weeks after these bacteria have appeared in the aquarium, the nitrite levels should drop to 0 and the nitrate levels will start to go up.
When the ammonia and nitrite levels have both gone up and then come back down to 0, and the nitrate levels start going up, the filter is considered cycled.
If your filter is only 3 weeks old, then you are only half way through the cycling process and probably have some bacteria eating ammonia and converting it into nitrite, but the second group of bacteria that eat nitrite and convert it into nitrate have not yet developed. This is why you have a low nitrite reading now.
The easiest way to reduce ammonia, nitrite or nitrate in an aquarium is by doing big daily water changes and reducing the food going into the tank. You should do a 75% water change and gravel clean the substrate any day you have an ammonia or nitrite reading above 0. And do a 75% water change if your nitrate level is above 20ppm.
Some countries have nitrates in their tap water and if you have 20ppm of nitrate in the tap water, then just do a 75% water change each week to try and keep the level at 20ppm.
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If your tank and filter are only 3 weeks old, do not clean it until it is at least 6 weeks old because you can upset the newly established bacteria and cause the cycling process to start again.
Once the filter has cycled, you wait a couple of weeks (usually around the 8 week mark), and then you can start cleaning the filter. Try to clean the filter at least once a month and preferably every 2 weeks.
If you have a power filter, sponge filter or box filter, you should wash the filter materials in a bucket of aquarium water. When they are clean, rinse them in a second bucket of tank water and then put them in the tank. Wash the filter case and impellor assembly under tap water. Then reassemble the unit and get it running again.
If you have an undergravel filter they gravel is the filter material and will be cleaned when you do a gravel clean.
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In your first post you said the fish has been unwell for several months. In your last post you said the tank has only been running for 3 weeks. Can you clarify that for me, how long have you had the fish and how long has the tank been running for?
If there is ammonia or nitrite in the water, it can damage a fish's fins and is the most common cause of fin rot.
The pleco does not appear to have whitespot. If you post pictures of the other fish I can check them but you can do it yourself too. Just look for small white dots on the fish's bodies and fins, they will be about the size of a grain of salt. Infected fish will usually rub on objects in the tank.
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Any filter that is running continuously in an aquarium for more than 2 months will develop colonies of beneficial filter bacteria that convert ammonia into nitrite, and nitrite into nitrate. This is the normal nitrite or filter cycle that we talk about in fish keeping. On average it takes about 4-6 weeks for this cycle to complete and during that time you get a build up of ammonia (caused by fish food, fish waste, rotting plants, dead fish, or anything else that breaks down in the water). After a couple of weeks you get colonies of good bacteria that appear in the tank and filter and they eat the ammonia and convert it into nitrite. The ammonia levels then come down and the nitrite levels start to go up.
When the nitrite levels start to go up, you get more good bacteria that start to grow in the filter and these convert nitrite into nitrate. A few weeks after these bacteria have appeared in the aquarium, the nitrite levels should drop to 0 and the nitrate levels will start to go up.
When the ammonia and nitrite levels have both gone up and then come back down to 0, and the nitrate levels start going up, the filter is considered cycled.
If your filter is only 3 weeks old, then you are only half way through the cycling process and probably have some bacteria eating ammonia and converting it into nitrite, but the second group of bacteria that eat nitrite and convert it into nitrate have not yet developed. This is why you have a low nitrite reading now.
The easiest way to reduce ammonia, nitrite or nitrate in an aquarium is by doing big daily water changes and reducing the food going into the tank. You should do a 75% water change and gravel clean the substrate any day you have an ammonia or nitrite reading above 0. And do a 75% water change if your nitrate level is above 20ppm.
Some countries have nitrates in their tap water and if you have 20ppm of nitrate in the tap water, then just do a 75% water change each week to try and keep the level at 20ppm.
--------------------------
If your tank and filter are only 3 weeks old, do not clean it until it is at least 6 weeks old because you can upset the newly established bacteria and cause the cycling process to start again.
Once the filter has cycled, you wait a couple of weeks (usually around the 8 week mark), and then you can start cleaning the filter. Try to clean the filter at least once a month and preferably every 2 weeks.
If you have a power filter, sponge filter or box filter, you should wash the filter materials in a bucket of aquarium water. When they are clean, rinse them in a second bucket of tank water and then put them in the tank. Wash the filter case and impellor assembly under tap water. Then reassemble the unit and get it running again.
If you have an undergravel filter they gravel is the filter material and will be cleaned when you do a gravel clean.
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In your first post you said the fish has been unwell for several months. In your last post you said the tank has only been running for 3 weeks. Can you clarify that for me, how long have you had the fish and how long has the tank been running for?
If there is ammonia or nitrite in the water, it can damage a fish's fins and is the most common cause of fin rot.
