Can i put normal garden weeds in a fish tank?

There are a couple problems with that. First, terrestrial plants will just rot if you put them underwater, making the water quality so bad it could poison your fish.

Secondly, even if you don't spray your plants with pesticides, there is probably runoff from places that do that get it on your plants, and that will poison your fish.

You need plants that are fully aquatic for aquarium use, preferably plants that you get a pet store. Second problem: Goldfish are not plant friendly. They eat most plants faster than they can grow, leaving you with a full goldfish and no plants. Goldfish also have a tendency to uproot plants planted in the substrate. You need bitter, hardy plants for a Goldfish tank. Some that may work are Java Fern and Anubias, which don't taste good and should be tied to decor with cotton thread rather than buried in the substrate. These are also hardy beginner plants and true aquatics that most pet stores carry.

Goldfish, by the way, will eventually need big tanks. Fancy Goldfish need 20 gallons for the first fish, and then 10 additional gallons for each additional Goldfish. Common and Comet Goldfish need ponds, or aquariums 75 gallons or larger. If you have tanks smaller than that for your particular Goldfish type, they probably won't live very long. They can live over 20 years if taken care of properly.

I would strongly advise you not to. Most plants can't live in just water or won't do well in fish water. However if you have a petco you can go and see if they have a plant called a marimo.they are usually about 6 dollars and are ardorable. They DO NOT float though the ones that are constantly floating are fake. I'll put in a picture of mine. It keeps the tank a little nicer makes it a little greener and is easy to care for. The weeds will rot and make the tank nasty and kill your fish but the a time is meant to live in water. As for household objects really there isn't anything that would be safe in a tank. But again petco has cheap 5 dollar ornaments just get ones that don't have sharp edges that could tear your fish's fins or hurt it's body.

no, they will just rot, foul the tank, and poison the fish

that's unless the garden has pesticide or herbicide residue in which case everything in the tank just dies.

go to a home center and see if they have a pond plant selection, try those instead, they're cheaper than the plants in aquarium stores. If you have adequate lighting they'll grow, otherwise they'll also rot and die, and foul the water and poison the fish.

floating duckweed which is usually not desirable because it smothers the tank, also is cheap and grows easily without as much light, you just have to remove half of it every few months to keep the tank clearer of it, and you need to put a screen on your filter intake and put the intake pipe lower in the tank to keep it from getting choked with duckweed.

as far as cheap decorations, most materials are harmful in fish tanks, with a few exceptions. Plain, uncoated, unpainted terracotta pots can be used, as well as clean white concrete. Soaking is necessary, at least a week of soaking and rinsing.

There's even recipes for making decorations out of white concrete and coral sand or plain "play sand". But it has to be rinsed and cured for months before use..!!!

most decorative sculptures are not made out of suitable materials, but some are. And using the concrete recipe you can recreate such things if you're really handy and have the skills to make molds etc.

Or the technique used to make "sand candles" can be used, where a mold of sorts is made by sculpting in a sand box and then pouring in the concrete mix. There's various techniques to make shapes and hollow structures by this method, but I'm running out of energy to keep writing, so further research on your part is necessary.

The only limitation is the size of your sand box and the level of your skills and engineering/technical knowledge and practice. You could conceivably make very intricate and large sculptures/rock caves and rock walls for the tank.

Check Garf.org and search for their Aragocrete recipe and hints. You will be using plain sand or a mix with coral sand, not Aragonite sand(which is expensive)

Large pieces of driftwood (soaked for weeks before use) are something you may be able to find, but other soft and hard woods are likely toxic or will rot.

Most plastics are ok, but you have to be sure there's no metal objects in them.

some online recipes and ideas for aquarium decorations, like expanding foam and paints, are not safe and should not be used.

I recommend you not to do so... Once, a friend of mine was in that same situation. Or you plant normal weeds inside it or you put artificial ones. Those normal garden weeds might not correspond to the water of the aquarium, and that will certainly affect your fish, but I don't know, I am not a professional, and I recommend you to ask in a proper pet store. But don't put them until you have absolutely sure that it'll be safe for the fish. Hope I helped.