Can a freshwater clam replace a tank filter?

NO. Reason is that they an Animal. Means they eat "stuff" and excrete waste. Sure they are "filter feeders", but that just means they take in small particles of food, algae, diatoms etc, digest it, and excrete waste, same as a fish, a frog, a snail etc.

Now plants can act as a filter system, because that Absorb waste from the water, and get their energy from light (photosynthesis). So a healthy plant can improve the water quality, and in some situations act as the filter system. A clam is on the other side of the equation.

Ian

Edit: A lightly stocked tank with snails and shrimp is a perfect candidate for plant filtering. The air stone keeps the water moving around the tank, and the plants are your filter system. So what you have may work just fine.

Let me put it this way: No.

Invertebrates such as clams (and shrimp and snails) are extremely sensitive to ammonia in the water. Breaking down ammonia is the most important reason for a filter in a freshwater tank. Nitrate is also toxic to mollusks, so you will also need to make large, frequent water changes to avoid a buildup of nitrates.

A clam, or maybe a lot of clams, might make the water clearer by filtering out particulates, but they wouldn't be removing dissolved pollutants, such as ammonia, from the water.

It's also not a good idea to replace an aerator with live plants. Here's why: Plants are net producers of oxygen and net consumers of carbon dioxide when the lights are on (or during daylight hours, if they are outdoors), because they are photosynthesizing, but in the dark they are net consumers of oxygen and net producers of carbon dioxide, because they don't photosynthesize in the dark. So if you have healthy plants, but no aeration, the carbon dioxide level in the tank will rise at night and the oxygen level will drop, potentially enough to asphyxiate fish (and shrimp and clams).

You could have the aerator going only at night, if you wish, though.

no,

clams can be used as a component of an ecosystem and will clean some types of debris from the water, but are not a replacement for a properly working filter.

clams are harder to keep alive than fish and require specific and well maintained conditions to survive..

in addition you seem to misunderstand the statement that plants "aerate" the water. They don't, and they don't produce enough oxygen to remove the need for a filter or "aeration" by other means. Although properly set up live planted tanks can be run without as much filtration or turbulence, adding one plant is not "a properly set up live planted tank".