I got no advice in the shop all I was told is you can have 4 of those however today I bought a 100 litre tank so I’m reading all I can
LOL, you have MTS (multiple tank syndrome) already and you've only been keeping fish for a short time. You're in trouble
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What sort of filter do you have on the tank?
Undergravel filters should be gravel cleaned every week with a basic model gravel cleaner.
Power filters and air operated sponge filters should be cleaned at least once a month. However, if the filter is less than 6 weeks old, do not clean it unless the water flow is really slow. After 6 weeks, the filter bacteria should have established and you can clean it once a month. To clean these types of filter, wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn.
If you have a filter with pads/ cartridges in and the manufacturer suggests replacing these every month, do not replace them until they start to fall apart. Replacing filter pads is a sales gimmick and causes you to lose the beneficial bacteria living in the filter.
If you have filter pads/ cartridges, buy a foam filter for another brand of filter, and use a pair of scissors to cut the foam so it fit sin your filter. Add the piece of sponge to the filter and leave it to run for 6-8 weeks before cleaning it. Sponges are cleaned as described above (by squeezing them in a bucket of tank water and re-using the sponge). Sponges last for years and only need replacing if they start to fall apart.
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If the tank is newly set up (less than 6 weeks old), keep feeding down to a minimum (2-3 times per week) and do big (50-75%) regular water changes (using dechlorinated water).