Okay, with that in mind you are going to have to jump through some more hoops but you have a few options. 1. Cut your water with r/o water. This will directly reduce the nitrate of the incoming water. Assuming you have well water your kh should be quite high (you are going to want to check this) I would suggest a 50/50 mix of ro water. 2. Zeolite - These are chips that you stick in a media bag in your filter. They will remove both ammonia and nitrate from your water supply. I've never personally used it but it's probably the easiest course of action for you. You could run the zeolite chips in a HOB filter on a garbage can or similarly sized container that you can use for water changes. It will pull the nitrate out of your water before it goes into your aquarium. This is the method that I personally would choose out of my three suggestions. 3. Plants - These actively remove nitrate much like Sekal said. However this is going to be the most expensive option. You will need a powerful light and appropriate fertilizers for your tank as well as a lightly stocked aquarium to combat the nitrate from your source water. Planted tanks however are the direction most people go with when they get addicted to aquariums
Start with a few 50% water changes to get your nitrate as low as you can (it wont drop lower than your source water) and then pick one of these and run with it. Sadly nitrate is the worst case scenario for things in your tap water