well you could compost the ground if you want to make a good dead to earth. i know peat is kinda expensive at times, yet there are recipes to make it yourself using leaves and good quality earth. be sure it doesn't have any trace elements, heavy metals or additives including fertilizers since it CAN kill your aquarium (nitrate is used as a fertilizer as well as ammonia) its the best way, or you could also gather rainwater, mix it with tap water and lower pH slowly that way. you could also add wood and allow the tennis to leach, but beware, tannis is a weak acid, yet wood has toons of it which could crash the pH. i would try on of this at a time, then mix em when you have an idea and are a bit more used to pH. but remember rainwater has No buffer an tap water depends were u live. so if u have a low buffer and wood with a lot of tennis your pH will crash DRAMATICALLY (personal experience from many years).
there are many other options to peat. tennis will also add that color ur looking for. rainwater won't. but please, don't add anything that doesn't naturally belong in an aquarium to the aquarium. unless you have vast knowledge on chemistry and have all the facts, you can never be too sure, and the risks vs benefits in this situation lean more to a risk with fewer benefits than wood or rainwater. evaluate your other options and research..
remember we are trying to re create the fish environment, if you subject fish into other environments, even if it can live, ill be in great stress and it will be detrimental on the long run.