Silly Questions About My Filter

You probably know all this, but for other people reading it -

The Fluval U series filters have four types of media.

The box in the centre contains biomax - white noodles. This is the main biological medium.
The box also contains a 'clean and clear' cartridge which is supposed to remove phosphate and nitrate. More on this below.
The sponges on either side of this box are both biological and mechanical media - they trap bits and the bacteria grow in them.
In between are poly carbon pads. These are a layer of filter wool and a carbon impregnated pad stuck together. They will act as a less efficient biomedium, a mechanical medium and a chemical medium.

The instructions say to replace one poly carbon pad and the clean and clear cartridge every month; and one sponge and half the biomax every six months. Don't.


The biomax (noodles) will last for years. Change half of them when they start to crumble, and the other half a month later. Just swoosh them in old tank water during a water change to remove the goo.
Sponges will also last for years. Replace them (one at a time) when they go into holes or won't go back to shape after squeezing. Just wash them by squeezing in old tank water.


Carbon removes organic chemicals in the water. It gets full and stops working so needs to be replaced regularly. But carbon is not necessary except for a few special occasions. It removes medication after treatment has finished; it removes the brown that leaches out of wood; and it removes any odd odours the tank may develop. You don't actually need to use carbon (you already know this  ). I haven't used any routinely for years. I do keep some in the cupboard in case I do need to use it.
You have a choice. Either just leave the pads in there and wash them; or replace them with plain filter wool, which does catch fine bits that go through the sponges. Filter wool can be replaced when it gets messy as it doesn't wash well. You can buy filter wool by the metre and use a pad as a template to cut it to size. Because it is replaced regularly and it doesn't make a good home for the bacteria, there will be very few bacteria on the filter wool when it is thrown away.
The main reason filter manufacturers use carbon is because it used to be the only filter medium back decades ago and for some reason these companies still want to use it. Maybe they are stuck in a time warp, or maybe they think their customers are.


'Clean and clear' cartridge. I must confess I'd not come across this until I just looked at the U manual. I thought the central box had just biomax inside.
It is supposed to remove phosphate and nitrate. Very few of these nitrate removing media actually work. And you only need to remove phosphate if your tap water has more than the recommended level as it promotes the growth of algae.
If this was my tank, since I don't have high phosphate, I would take out this cartridge and put more biomax in its place. Or even a plain sponge - if the other sponges are fine sponges (small holes) I'd get a coarse sponge, or vice versa. Sponges are a very good biomedium.



I also have a small cheap filter and a mesh bag in the cupboard. Should I need to use carbon, I use granules in the bag inside this filter. And if I did have high phosphate, I'd probably use a small filter for phosphate removing media as well.

Thank you 

The main message about filters is that you need something to catch bits (mechanical media - usually sponge) and something for the bacteria to live in (biomedia - usually sponge and/or ceramic media). And to ignore the instructions that say to replace them on a regular basis.

Carbon is not needed, and ammonia removing media such as zeolite are not a good idea. The problem with this is that so many of the mini filters have nothing but a cartridge containing carbon, and sometimes zeolite as well 

Thanks very much for the replies!