what is an anode?

Any time you have two different metals that are physically or electrically connected and immersed in seawater, they become a battery. Some amount of current flows between the two metals. The electrons that make up that current are supplied by one of the metals giving up bits of itself — in the form of metal ions — to the seawater. This is called galvanic corrosion and, left unchecked, it quickly destroys underwater metals.
Common casualties of galvanic corrosion are trim tabs. The way we counteract galvanic corrosion is to add a third metal into the circuit, one that is quicker than the other two to give up its electrons. This piece of metal is called a sacrificial anode, and most often it is zinc or aluminium. In fact, most boaters refer to sacrificial anodes simply as zincs.
It would be hard to overstate the importance of maintaining the anodes on your trim tabs. When an anode is missing or largely wasted away, the metal component it was installed to protect begins to dissolve.
When purchasing your trim tabs, remember to buy anodes.

Why Aluminium?

Aluminum anodes tend to corrode more uniformly than other metals, which means that they provide more reliable and predictable protection for the metal they are intended to protect. This is important in applications where even minor damage to the structure being protected can have significant consequences.

Aluminum anodes are better than magnesium anodes in fresh water because the environment in fresh water is less corrosive than saltwater, and aluminum anodes are more effective in such an environment.

In freshwater, the lower conductivity and lower salt content make the environment less corrosive than saltwater. Additionally, the pH of fresh water is generally higher, which also contributes to a less corrosive environment. In such conditions, aluminum anodes are more effective than magnesium anodes because they corrode more slowly, which means they last longer and provide better protection for the metal they are supposed to protect.

Furthermore, aluminum anodes are less likely to cause damage to boats and other structures than zinc or magnesium anodes. This is because magnesium anodes are more active and corrode more aggressively than aluminum anodes, which can cause pitting and other forms of damage to the structure being protected.

Another advantage of aluminum anodes is that they have a higher current capacity than zinc or magnesium anodes. This means that they can protect a larger area of metal from corrosion than magnesium anodes can.

In summary, aluminum anodes  are more effective in a less corrosive environment, are less likely to cause damage to structures & have a higher current capacity.