If you are putting a pleco in there (I assume that's what you mean and you had a typo?), no salt. Plecos are armored catfish, which means they have plates instead of scales, and because of this salt irritates their skin.
Nitrate is removed by partial water changes. As long as you don't have Nitrates in your tap (a few places do, but most do not) partial water changes will be far more effective at preventing Nitrate poisoning anyway. All of those fish have high bioloads, so change 50% of your water a week, and that should keep Nitrates down. Or you could do 2 30% water changes twice a week. This needs to be done to keep the tank clean anyway.
If you do have Nitrates in your tap and water changes won't work as well you could try getting some live plants. Goldfish will eat plants, but there are some that are so tough and distasteful that they will leave them alone. These include Cryptocoryne, Anubias, and Java Fern, all of which are hardy low-light plants that won't need much extra attention. Plants will also consume some other chemicals that are bad for fish in the water, and provide some dissolved oxygen as well.
Less effective than water changes or plants, but still some help are filter inserts that absorb Nitrates. These must be changed regularly so that they don't max-out and then start releasing Nitrates back into the water.