I have below in a 100 litre tank
Platy - 2
Fluorescent Tetra - 6
Koi Carp - 4
Tiger Barb - 4
Black Molly - 3
White Molly - 1
Orange Molly - 3
Should I isolate the other platies as well?
I usually do 25% water change every 2 weeks and change filter at the same time.
You don't need to isolate the other platies because the disease is in the main tank and all the fish have been exposed to it already. Doing big daily water changes and gravel cleans for at least 1 week, will dilute the disease organisms and reduce the chance of it killing other fish.
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The Koi carp need to be returned to the shop or put in a big fish pond. Most Koi carp will reach 12 inches in length within 2-3 years and they can grow to more than 3 feet long.
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The platies and mollies do best in water with a pH above 7.0 and a GH of 200ppm for the platies, and 250ppm+ for the mollies.
The tiger barbs and most tetras do best in water with a pH around 7.0 or below, and a GH below 150ppm.
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You have a few fish in the tank and need to do a bigger water change and gravel clean the substrate every week to keep the water clean. I normally recommend doing a 75% water change every week to help keep nutrients down and to dilute the disease organisms in the water.
As mentioned by Essjay, filter media should be cleaned and re-used, and not thrown out every few weeks. The filter media/ material becomes home to good bacteria that help keep the ammonia and nitrite levels at 0ppm. If you replace the filter media. you get rid of the good bacteria and end up with water quality problems that kill the fish.
New filters should not be cleaned for the first 6-8 weeks. This allows the good filter bacteria time to fix themselves onto the filter media. After that time you can clean the filter at least once a month, and every 2 weeks is fine. But when you clean the filter media, you should just squeeze the media out in a bucket of tank water until it is clean, then squeeze it out in a second bucket of tank water. Leave the filter media in the second bucket of tank water while you rinse the filter case under tap water. Shake the tap water off the filter and then put the cleaned media in the filter and set it back up and turn the filter back on.
If the filter media starts to break down or fall apart, then replace it. You can buy sponges for various brands of power filter, and use a pair of scissors to cut the sponge to fit in your filter instead of using replacement filter pads/ cartridges. Sponges last for years and get squeezed out in a bucket of tank water and re-used.
The bucket/s of dirty tank water get poured on the lawn/ garden outside.