The bacteria live on whatever substrate that there is sufficient water motion over. The more surface area that a solid object has the more room there is for bacteria to grow. Remember the smaller the particle size is the more surface area it has. As for filtration here's some general definitions.
Under Gravel Filters (UGF). These can be driven by airstones or by a powerhead pump. These filter by pulling the water down through the gravel and up through the tubes. These offer a large surface area but care must be taken in keeping them clean and keeping the flow rate adequate.
Reverse under gravel Filters (RUGF). This is the same setup as the UGF except a reverse powerhead is used to push water down the tubes and up through the gravel. This offers all the benifits of the UGF without the debris buildup. Usually a filter is needed before the powerhed to catch large particles.
HOB (hang on the back) filters. These filters usually offer less surface area for bacteria but have the highest flow rates. If you get one of these look for one with a lot of area for a sponge or biowheels.
Canister filters. These filters offer less flow than the HOB but they have much more for sponges, rings etc for bacteria to grow on. These are generally considered the best for long time service.
Canister filter + RUGF. This offers the highest filtration available. The canister filter is used as the primary filter. The outlet of the canister is then attached to the tubes of the UGF. This sends filtered water up through the gravel to remove the harmful waste products.
My suggestion is to buy equipment that you can use when you upgrade the tank.
If you want to, right now you can use a classic UGF with your existing HOB. Just be sure to do weekly gravel vacuums and 30-50% weekly water changes. But when it comes time to upgrade to a 55 gallon or larger tank you might have to buy new equipment. Depending on the shape of the tank the UGF plates might not work in your new tank. I personally would buy a canister filter at this point.
Under Gravel Filters (UGF). These can be driven by airstones or by a powerhead pump. These filter by pulling the water down through the gravel and up through the tubes. These offer a large surface area but care must be taken in keeping them clean and keeping the flow rate adequate.
Reverse under gravel Filters (RUGF). This is the same setup as the UGF except a reverse powerhead is used to push water down the tubes and up through the gravel. This offers all the benifits of the UGF without the debris buildup. Usually a filter is needed before the powerhed to catch large particles.
HOB (hang on the back) filters. These filters usually offer less surface area for bacteria but have the highest flow rates. If you get one of these look for one with a lot of area for a sponge or biowheels.
Canister filters. These filters offer less flow than the HOB but they have much more for sponges, rings etc for bacteria to grow on. These are generally considered the best for long time service.
Canister filter + RUGF. This offers the highest filtration available. The canister filter is used as the primary filter. The outlet of the canister is then attached to the tubes of the UGF. This sends filtered water up through the gravel to remove the harmful waste products.
My suggestion is to buy equipment that you can use when you upgrade the tank.
If you want to, right now you can use a classic UGF with your existing HOB. Just be sure to do weekly gravel vacuums and 30-50% weekly water changes. But when it comes time to upgrade to a 55 gallon or larger tank you might have to buy new equipment. Depending on the shape of the tank the UGF plates might not work in your new tank. I personally would buy a canister filter at this point.
