There might be a risk, but if you're not sure, you could treat the new fish with melafix once you introduce him That should do the trick! Since most fish are weakened by stress when they move to a new home, it's good to feed them a diet rich on c-vitamin for their immune system, or to use natural meds like melafix, that doesn't interfere with the bacteria in the tank

Any tank WILL NOT EVER be instantly cycled. It always takes a while for a good amount of beneficial bacteria to establish. So even by adding substrate/water from an established system, it will only speed up the process a bit at most. It's a safer method though than adding fish right away
And I wouldn't put filter media under the gravel, I can't help but imagine it being a mess, so I would rather take some water from your goldfish tank and put that into the betta's home. Then you can monitor the water quality, and you'll have a nice starting point! There you do the same, regular water changes. And once the tank's cycled, you can add your betta along with melafix
I don't know how to dose melafix, but it should say somewhere on the bottle something like "x drops/ x ml per x gallons of water" Then you can easily calculate the correct amount if it says for example "5ml per 3 gallons" by dividing the amount by three. There you have dosage for one gallon!

I say your betta will be delighted! Most people simply put them in a bowl (which they easily survive as well) without heater/filtration. This is possible because bettas have a labyrinth organ that allows them to breathe air right from the surface, as they live in shallow, muddy pools of water in nature. Just like guramis, they're a part of the family Anabantoids. So if your air stone will take too much space, you can easily leave it out
I'd say you'd be fine even without the heater d: It usually doesn't get very cold indoors.

actually suprisingly the media isnt a mess at all .. and its a biobag filter in the first tank and a UG filter in the one gallon ( which I am not establishing yet)

And it'd be 1/10 of a teaspoon ... how do i measure that?

1 Teaspoon [US] = 4.9289 milliliter

and its much less than an ounce. The entire bottle is 8 ounces lol

no it doesnt but 1/2 a ml should be good so yeah a syringe ... But i cant set up the tank yet. I want to get my first tank set up right before I go through cycling again with a new tank and buddy

Lol Its fine what country are you in?

And I retested this morning. Ammonia .25 ... I cant get it to come down

I'm from Finland c: So I mainly use European measurements, but I'm used to thinking in gallons and inches lately d:

Hi Elli,

keep doing waterchanges and add plenty of beneficial bacteria.

The detoxifier you use will not remove the ammonia, etc. It only makes it non toxic for your fishies.
So you will have readings of it.

Ahhh, yes and there are U.S gallons and U.K gallons, I am always confused using gallons.
I just love our litres, LOL, but I started to get used to inches slowly, haha

i like warm lol And lakes and I LOVE the ocean

And it says *removes* The others said detoxifies ... mean lying bottle lol

LOL I love extremely hot but it seems lately my body is changing and likes cold .. makes no sense to me

Photos say so much.  Observations I make from seeing your set up are as follows.
1.  The large glass pebbles will let sinking food fall out of reach from the fish where it will go rotten and grow bacteria contributing to pollution problems. Sand or fine gravel that the fish can pick up, clean/eat and spit out is more natural. Also fine gravel has a larger surface area than large pebbles which means a larger biological system and a greater amount of lower form algae on the gravel that the fish can pick at.

2. The water is so clear it seems cloudy white not even the slightest tinge of green or crystal clear pond type water.  I think you have chemicals in the water which are not required and making too many water changes.

3.  The live plants in the tank are very few and looks to be Egeria which goldfish will not touch even if starving. Live plants in a tank will feed on lower plant forms (i.e. suspended microscopic algae, pea green water) by filtering. Floating Duckweed is one plant which goldfish love to eat and it's very healthy for the fish.

4.  Goldfish are a very strong fish, naturally liking murky water and easily tolerate low oxygen levels.  I would change the gravel and let the water age. Let the natural system kick in.

I'm not sure if it's a very good idea to put gravel on top of the marbles. The dirt will be trapped there for sure now.
So if I was you, I wouldn't add the gravel until the goldfish are out, and you could clean the tank thoroughly to get rid of all the waste hidden in the glass pebbles. Then you could keep a few of the pebbles on top of the gravel as some decorations or something d:

Ah, and the plant is Elodea btw. Commonly called Canadian pond weed

my goldfish wont touch it. And I dont like the marbles at all ... Plus I dont want to waste all this cycling by emptying the tank and redoing everything. Is there another way to get all the poop out? I dont have a gravel vacuum yet.

I looked up pictures and its anacharis ... I'm so confused They nibble at it but dont overly seem interested

Yep, Anacharis, Elodea, it's the same thing.... haha like "hungry dogs" lol.... I had 2 lovely lush bunches of Ambulia and they chomped it down to the stumps and pulled on it like a starving lot of hungry LIONS.... . Almost nothing left after only ONE week.