Goldfish actually need plants in their diet, that's why they eat them.
If you have a light unit above the aquarium and add a variety of plants, hopefully most of the plants will grow and tolerate being grazed on by the goldfish. Some good plants to try include: Ambulia, Hygrophila species, Ludwigia species, narrow Vallis, and common Amazon Swordplant, with duckweed and Water Sprite (Ceratopteris thalictroides/ cornuta) to float on the surface.
Ambulia and Water Sprite have soft leaves and the goldfish will eat them along with the duckweed, but it is beneficial to their overall health.
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If you have soft water (water with not much mineral content) then you could add a couple of pieces of limestone rock to the tank to help keep the pH above 7.0 and to increase the hardness of the water a little bit.
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How big is the tank?
Small tanks that are less than 3 foot long are not really suited to goldfish with a single tail (common goldfish, comets & shubunkins). And if the tank holds less than 60 litres you probably want to stay away from goldfish in general because they need a bit of room due to the size they can grow to.
Do not mix goldfish with different numbers of tails. Either keep single tailed fish or fantail (double tailed fish). The fantails have shorter bodies with 2 tails and can't compete for food with the single tailed fish that are much more agile.