Here's why:
* Reduced surface area for beneficial bacteria: Undergravel filters rely on beneficial bacteria that colonize the filter plates. Covering the entire bottom of the tank limits the surface area available for these bacteria to grow, reducing the filter's effectiveness.
* Decreased water flow: A fully covered bottom creates a stagnant area where water flow is reduced, leading to poor water circulation and potentially harmful buildup of waste.
* Difficult cleaning: It's very difficult to clean the entire filter without disrupting the entire tank. This can be stressful for your fish and may even damage the filter plates.
* Limited substrate options: Undergravel filters require a specific type of gravel that allows water to flow through it, limiting your choices for other substrate types.
Instead of covering the entire bottom, consider:
* Placing the filter plates strategically: Place them in areas where you want to encourage water flow, like near the intake of your power filter.
* Using a smaller area: Cover only a portion of the tank bottom, leaving space for other substrate types and creating better water circulation.
* Considering alternative filtration methods: There are many other types of filters available, such as canister filters, hang-on filters, and sponge filters, that may be more efficient and easier to maintain.
Remember, the key to a healthy fish tank is proper filtration, and an undergravel filter may not be the best option if you want to cover the entire tank bottom.