The goal for the filter would be to keep the water pump clean, so normally you would place the mechanical filter media where the water enters the filter. The water should not be able to go around it, but instead have to pass through the media. Mechanical filtration pulls out pieces of things. If you accidentally get too much food in the tank, and it makes it to the filter, this is what you want to pull out and clean at the faucet. Remove the debris, run some tank water through it so you remove the chlorine, then put it back in the filter. Carbon/charcoal and sometimes white rocks called zeolite will be doing your chemical filtration, removing heavy metals, and other chemicals, so they can go next. The biological filter media is what is most important. Have this media - a large amount of surface area that water moves through, but will not get clogged up - next to the pump part of your filter. NEVER change these. These are the home for your beneficial bacterial cultures. Detritus (decaying organic material, solid waste, uneaten food, dead plant matter) and liquid fish waste become toxic ammonia. These bacteria consume the ammonia and turn it into toxic nitrites. Another bacterial culture living in the same place turn toxic nitrites into nitrates. Nitrates are plant food, here comes your algae if you leave your lights on all day but don't have live plants in your tank. Nitrates gradually build over time, and fish gradually become accustomed to higher levels of nitrates. This is why we do Regular Partial water changes, to keep nitrate levels low. If you don't do Regular Partial water changes, and suddenly change a lot of water, or add a new fish that doesn't get the long period of time to acclimate to these higher levels, fish can get sick and some may die. Biological filtration is the most important thing in your aquarium, second only to water. Know where your bacteria live and DO NOT clean it, replace it, dry it out, bleach it, wash it under tap water, and so on. Just leave it alone the same way you leave your fish alone. Fish can survive just fine without mechanical and chemical filtration, so there is really no need to continue to use those in your aquarium, other than keeping your money in the pockets of the people who tell you that you have to have them and you have to replace them. Any company that tells you the biological filter media must be replaced is lying to you and I would urge you to never buy their products again. A bulk amount of carbon and nylon mesh bags can be purchased from any decent pet store or supplier, as can foam that can be cut to fit into your particular filter. Personally, I recommend not bothering with either of these, and adding more biological filter media in the extra space you now have. You can never have too much beneficial bacterial in your tank.
Answers & Comments
The goal for the filter would be to keep the water pump clean, so normally you would place the mechanical filter media where the water enters the filter. The water should not be able to go around it, but instead have to pass through the media. Mechanical filtration pulls out pieces of things. If you accidentally get too much food in the tank, and it makes it to the filter, this is what you want to pull out and clean at the faucet. Remove the debris, run some tank water through it so you remove the chlorine, then put it back in the filter.
Carbon/charcoal and sometimes white rocks called zeolite will be doing your chemical filtration, removing heavy metals, and other chemicals, so they can go next. The biological filter media is what is most important. Have this media - a large amount of surface area that water moves through, but will not get clogged up - next to the pump part of your filter. NEVER change these. These are the home for your beneficial bacterial cultures. Detritus (decaying organic material, solid waste, uneaten food, dead plant matter) and liquid fish waste become toxic ammonia. These bacteria consume the ammonia and turn it into toxic nitrites. Another bacterial culture living in the same place turn toxic nitrites into nitrates. Nitrates are plant food, here comes your algae if you leave your lights on all day but don't have live plants in your tank. Nitrates gradually build over time, and fish gradually become accustomed to higher levels of nitrates. This is why we do Regular Partial water changes, to keep nitrate levels low. If you don't do Regular Partial water changes, and suddenly change a lot of water, or add a new fish that doesn't get the long period of time to acclimate to these higher levels, fish can get sick and some may die.
Biological filtration is the most important thing in your aquarium, second only to water. Know where your bacteria live and DO NOT clean it, replace it, dry it out, bleach it, wash it under tap water, and so on. Just leave it alone the same way you leave your fish alone.
Fish can survive just fine without mechanical and chemical filtration, so there is really no need to continue to use those in your aquarium, other than keeping your money in the pockets of the people who tell you that you have to have them and you have to replace them. Any company that tells you the biological filter media must be replaced is lying to you and I would urge you to never buy their products again.
A bulk amount of carbon and nylon mesh bags can be purchased from any decent pet store or supplier, as can foam that can be cut to fit into your particular filter. Personally, I recommend not bothering with either of these, and adding more biological filter media in the extra space you now have. You can never have too much beneficial bacterial in your tank.