Thanks for providing the tank dimensions (4ft long x 18inches wide x 21inches high).
Not everyone on the forum uses gallons for tank sizes and tanks can vary in shape, so a 75 gallon tank could be 6ft long x 1ft wide or it could be 3ft long x 2ft wide. This is why we ask for dimensions rather than gallons.
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What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply?
We need this information in numbers and measurement units so we can make suggestions for species that suit your water conditions.
This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
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Will you be keeping your current guppy tank?
If yes, you could keep guppies in one tank and have tetras and other fishes in the bigger tank, assuming the GH and pH is suitable for them.
When you do eventually upgrade to the new tank, you won't have to cycle it. Set the new tank up with gravel, plants, ornaments, filter and dechlorinator. Let it run for a day or two and then move some of the filter media from your current tank into the new tank's filter. Wait 24 hours, check the ammonia and nitrite levels, if they are good, add some fish.
If you aren't keeping the guppy tank running, then set up the new tank as described above. Test the water 48 hours after setting it up. If there's no ammonia or nitrite, move the contents of the guppy tank (filter, fish, snails, plants) into the new tank and your done. Keep feeding down for a week or so and monitor the water. If everything is good after 2 weeks, get some more fish.
You could use the guppy tank as a quarantine tank for new fish so they don't introduce diseases into the established tank.