Petco: Select Fishkeeping Supplies

So first, what kind of fauna does she want to have? 2.5 - 5 gallons are good for shrimp/snails, small school of endlers, pea puffers, or pygmy cats. Putting a betta in anything smaller than a 10g is cruel, and many will tell you so. Yes, they are sold in small cups, but that is a tactic to help them flair up and color up being next to other males and get them sold. They will live longer and alot happier in a tank they can actually swim around in.

As for the filter, the 3 stages consist of the sponge, the media and the carbon. The carbon is trash after about a month or so depending on what else you may add to the water. Its main job is to clarify the water and remove toxins, but it doesn't have a long life. I personally don't use it in any of my tanks. The media is the life support system. This will allow the beneficial bacteria to grow and thrive in your tank. Lastly, the sponge collects all the debris/detritus, which should be cleaned monthly in small tanks. With a monthly cleaning, the filter will last you plenty. The small pump that they come with needs to be cleaned monthly as well to avoid clogging. They are very easily replaced and affordable if they ever die on you though. I personally stay away from the Aqueon tanks. Their filters are not that great, and they tend to lean on the cartridge base filters, which not only is a waste of money, but it hurts your bacteria load every time you throw one of the cartridges away. The Fluval brand has been solid for me. I personally own the Fluval Flex V, and the Flex 15. Both are very ascetically pleasing, and have a decent build to them. There are also lots of mods you can do to their filtration if you look around on Etsy. As for keeping the tank livable, you just have to make sure you cycle it well (look up nitrogen cycle on YouTube), and test the water before adding any life to it. And when you clean the tank/filter, you clean the media in tank water and not water out of the tap. The chlorine in tap water will kill all your bacteria, and you will have to start all over again. Every time you do water changes, you re-add water conditioner/dechlorinator. At the end of the day, fish are just like every other pet that need to be cared for in order to maintain a healthy, happy life. Here is my 125G Planted Tank. [reddit.com]

Do yourself and your fish a favor and get at least a 10g aquarium. Bigger = stability