* Fully aquatic: Spends its entire life in the water.

* Non-carnivorous: Eats only plant matter.

* Compatible with Neon Tetras and Siamese Fighting Fish: Doesn't pose a threat to them and shares similar water parameters.

Unfortunately, there is no frog that perfectly meets all these criteria.

Here's why:

* Fully aquatic frogs: Most frogs are semi-aquatic, meaning they need both land and water. Fully aquatic frogs are less common.

* Non-carnivorous frogs: Most frogs are carnivorous or insectivorous. True herbivores are rare.

* Compatibility with neon tetras and Siamese fighting fish: Even if a frog is herbivorous, it may still pose a threat to small fish, especially if they are young or stressed.

Here are some potential alternatives:

* African Dwarf Frogs: These are fully aquatic and mostly herbivorous, but they can nibble on small fish if they get hungry. They might be okay in a tank with larger neon tetras but not with Siamese fighting fish.

* Pacman Frogs: These are fully aquatic and carnivorous, but they can be trained to eat commercially prepared frog food, which is mostly plant-based. However, they are large and might be a threat to smaller fish.

It's important to note that:

* You should always research the specific needs of any frog you're considering.

* You should observe your frogs and fish closely for signs of aggression or stress.

* It's crucial to provide a large enough tank with plenty of hiding places for all your animals.

The best option for your specific situation might be:

* To choose a different type of frog: A semi-aquatic frog that spends most of its time on land would be less likely to bother your fish.

* To focus on a tank dedicated to fish: This would allow you to create the ideal environment for your neon tetras and Siamese fighting fish.

Remember, responsible pet ownership requires researching and understanding the needs of all your animals before bringing them home.