General Questions About Adding Pets

Question
Hello.
I have a fairly large tank, a 26 Gallon Bow-front by Aqueon. I currently have 2 Plecos, 1 Pictus Catfish, 1 White Mystery Snail, 3 Gouramis (Gold, Blue, Pink), 5 Tiger Barbs, 3 Tetras (2 Flame, the other is unknown ATM), and 1 Dinosaur Eel.
The tank currently has a gravel base, a small bubbling volcano, a large rock formation (fake, but filled with many holes), and a small castle right up front. In addition, i have multiple plants growing within the tank. I Change the water every 2 weeks, and the filter whenever the water begins to get cloudy and yellow. My water quality appears to be good. However I can not always tell and just use my best judgment, I will add PH correction tablets, ammonia neutralizers, and other such things when i change the water. I also add a layer of Stress Coat and Stress Zyme+ periodically as well.
My questions revolve around adding more specimen to my tank. No Fish however, but more along the lines of Frogs and Crabs. What should I do, if i even should, Add those animals to my tank.

Thank you very much.

Answer
Crabs Eat smaller fish (tetra) and need a place to get out of the water, aswell as some species who need salt water..(I wanted a crab)

Frogs can get attacked by fish, for example, gourami's and somme species of frog need LARGE tanks as they get big and will start eating smaller fish..aswell as a place to get out from the water..apart from one species who doesnt need that

hope this helps