Koi Fish Information Centre

Noura,
Often it is called glass wool, which is a throwback to earlier days when it was actually fiberglass, and called glass wool. Today, it is polyester, for the most part.
With the layer of glass wool between the gravel and the UGF (UnderGravel Filter), the health of your filter becomes problematic. Eventually, that layer will become clogged, and there is no good way to clean it without completely removing the substrate and replacing it. The failure of this layer will probably take some time to be discovered, which will not be good for your fish. When it is convenient, you should remove this glass wool from your set up. After this has been done, you will need to occasionally put a tube down under your UGF to siphon off the mulm that will collect there.
When you change the way this filter works, you are likely to suffer a mini-cycle from the loss of nitrifying bacteria, but keeping the existing gravel damp will definitely help minimize this.
For your other filter, I’d replace it with an external hang off the back (HOB or HOT (Hang Off the Tank)) that uses foam blocks as the filter media. Use of granular activated carbon (the coal you refer to) or GAC, is only recommended in certain situations for removing such as medications or other dissolved organic carbons (DOC).
For your two gallon, I’d switch you over to a sponge filter, which will do all your current bubble up does, and it will also provide a food source for your fry.
The last two suggestions will slightly increase the volume of water in your tanks, which is very important for the way you are stuffing fish in those tanks. Look around your home with a bit of imagination, and I am sure you will find space for more tanks.
\Steve//

Category: Philippines Internet Koi Society