With the filter cleaning thing, just clean out the sponges when they become quite full of muck and effect the flow of the filter- for some people this is once a week, for others it can be over 3 weeks- a lot depends on the tanks stocking.
When you clean your filter sponges out, just clean them roughly out in some old tank water- never clean them so they are sparkling clean (as the beneficial bacteria needs a little muck to live off, over-cleaning the sponges can cause the tank to mini-cycle, so you just need to clean the sponges enough so the filter runs smoothly with a strong current) and never clean them out in tap water (as the chlorine in the tap water will kill off the beneficial bacteria in the sponges). Old tank water is the best for cleaning filter sponges out in as it is least likely to upset the beneficial bacteria in the sponges, so just save a bucket or two of it when you are doing your water change to clean the filter sponges out in
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The black sponge in the juwel filter range is the carbon sponge- this removes chemicals like fish medicines from the water. This is good when you've just finished a course of medication/treatment for your fish and you want to remove the meds from the water, but other than that the sponge is pretty useless for much else. When using any meds on fish, you should always take the carbon sponge out of the filter as you don't want it to absorb all the meds you put into the tank and stop them from working.
Carbon sponges stop working after a couple of months (i would say they are active for about 6months maximum, but there are a lot of factors involved when it comes to how long they'll work for, more than often they last a lot less) and if they are used after their ability to absorb chemicals has stopped, they will leak out all the chemicals they have absorbed back into the tank which can be very bad for the fish as if you've used a lot of meds and can end up with the result of overdosing the fish with old chemicals.
So the best way to use a carbon sponge is to only have it in the filter when you actually need to use it as that way it'll last longer if its not in constant use and be safer and more effective too when in use.
With the thin fine white sponge, this is just there to help clean up any very fine waste in the tank and help keep the water clearer/cleaner looking in general. I have one in my main goldfish tank filter, but i don't use it all the time as it gets clogged up very quickly with all the poop being produced by the goldfish in the tank- sometimes its just better to have a much courser sponge more suited to dealing with lots of big fish poops and muck in general when keeping high waste producing fish like goldfish or pleco's.
These fine white thin sponges though don't last very long though in comparison to other sponges- i find that mine will last up to 6-9 washes before getting too knackered out, however they do last a lot longer if you wash them gently than what a lot of people realise.
The finer the sponge the more surface area it has and thus the better it is for your beneficial bacterial filtration in the filter as the bacteria lives on surface and thus the more surface area the sponge has, the more beneficial bacteria it can grow on it.
However the finer the sponge the less able it is to deal with lots of waste and fine sponges tend to get clogged up too quickly by waste to be of much good. The bacteria thrives best off the finest and oldest waste in the sponge which is like a sludge, waste which is course and which you can still make out what it consists of (like old dying plant matter) is not that great for the bacteria to live off as it needs to decompose more to be of much good as food to the bacteria.
So to get the most out of your filter, its best to have at least two types of sponges in it- one which is course/has large holes and is good for the mechanical filtration of the tank, and one which is fine/has many small holes which is good for the biological filtration of the tank
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