Welcome to Tropical Fish Keeping forum. Nice to have you with us. You will find lots of friendly aquarists here, many with considerable experience and some fairly new.
I am not familiar with that specific filter, but generally speaking the filter media does not need to be "replaced" but just rinsed to keep it clean so the water continues to easily flow through it. At the water change, siphon out some tank water into a pail and use this to rinse the media. Tap water will kill the nitrifying bacteria that attach to the media.
A partial water change should be done every week. The volume can vary depending upon the aquarium's biology. This is a 15g and you have a fair number of fish--you may think it looks "a little empty" but small fish will grow quickly if they are healthy and in a suitable environment, and the tank must always take this into account. The mature size of any fish should always be considered when selecting them, and only if you now have what they will need then should you acquire the fish. This is the safest way to ensure you will be able to provide a good environment. So I would recommend changing approximately 1/3 to 1/2 the water volume weekly. Use a good conditioner. This will provide stable water conditions long-term.
Now to the fish. The shark is going to be trouble. Presumably this is either the
Red Tailed Shark or the
Rainbow Shark; the latter attains 6 inches and the former 5 inches, and both require a 4-foot tank at minimum. We have fish profiles, second tab from the left in the blue bar across the top of the page, and if the name is used in posts the same it will shade and you can click on the shaded name to see that fish's profile. Please have a read, the info there will explain the dangers. I would seriously recommend returning this fish. In such a small (to the fish) environment, it will likely become even more aggressive as it matures, and other members can tell you how one of these can kill the other fish.
The tetra and hatchetfish are shoaling fish. They live in large groups and must be kept in a group in the aquarium. There are many factors in this, but suffice it to say that without a sufficient group they will be stressed and this can cause ill health and aggression, even in otherwise peaceful fish. These species are also in our profiles. Click on
Bloodfin Tetra and
Silver Hatchetfish to read about their needs and numbers for the group.
The
Guppy might get nipped by the bloodfin; as noted in the profile, this species is prone to fin nipping, and the colour fins of a
Guppy might be too tempting.
Increasing the tetra and hatchetfish numbers is needed, and removing the shark. This will complete the 15g.
Byron.
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