Here's why:
* Betta: These fish are very hardy and can tolerate low water flow, making them suitable for smaller tanks without filtration.
* Guppies: While they're generally more active than bettas, they can also do well in tanks without filters, especially if you keep the water clean with frequent water changes and have a low stocking density (not too many fish).
However, it's important to note:
* Filtration is still highly recommended: Even though these fish can survive without a filter, it significantly improves water quality and reduces the workload of regular water changes.
* Size matters: For both betta and guppies, the smaller the tank, the more important it is to have filtration.
* Other factors: The number of fish, their feeding habits, and the type of substrate (gravel, sand, etc.) can all influence the need for filtration.
Here's a general rule of thumb:
* Small tanks (under 5 gallons) for betta and guppies: A filter is highly recommended.
* Larger tanks (5 gallons and up): You can get away without a filter if you keep the tank very clean with regular water changes, but a filter is always a good idea.
Bottom line: While betta and guppies don't absolutely need a filter, it's highly recommended to use one to ensure optimal water quality and the health of your fish.