Actually they are not essential. My uni has a tank that has just a bunch of rocks in it. Also guppies and cories, it is generally cloudy but water conditions seem to be okay. There are at least 100 ramshorn snails and the substrates is literally COVERED in cherry shrimp. I honestly wouldn't worry about filters, just clean it weekly. I take at least a dozen shrimp out of my filter and they are all alive and fine. Just toss them back in the tank.