No guppies, do not add anything until you know what's wrong with the pleco. After that, some fish have to go before you add any more fish.
You need a heater for tropical fish, which is what cories are. A heater will keep the temperature from fluctuating, which is stressful for the fish.
Do you have a filter in the tank now? If not, air stones will help, but you need some sort of filtration for beneficial bacteria to grow. This tank is not cycled. Read the article on cycling.
You need to be doing 50% water changes daily to get the nitrites down. You will need to be doing water changes often if you do not have a filter.
No garden bricks, unless you can be 100% sure there are no chemicals, soaps, detergents on it.