what are the best conditions for a crayfish?

I would put a few in a 5 gallon tank. Filtered with a small current to mimic the creek. That's good. Use the same water from the creek and put a thermometer in the water. Check how hot or cold it is. Buy a heater that could go to that temperature.

I would let it go, they are hard to care for properly for the inexperienced.

Crayfish need a minimum of 10 gallons of water, and need to be kept alone with many rocks and hiding places. They are not beginner inverts at all, and need great water parameters to thrive.

they prefer sand or a fine grain gravel. They need highly oxygenated water and after you have a heater and bubbler in, the openings must be sealed as they are notorious escape artists. They will literally climb up the filter and get out. They enjoy live plants, but they tend to eat them.

They need very clean over filtered water. Most need a heater, but many North American species don't. Do your research! Their tank needs to be cycled just like with fish, and water should be treated with conditioner for every water change. Water changes of at least 25% should be performed weekly or biweekly.

Also, they will need liquid calcium. The same kind you might get for stony coral, this will help with molting and keep their exoskeleton strong.

They should have a wide and varied diet. Pellets and wafers for carnivorous fish along with fresh veggies like zucchini are good for them. They can be fed live food once in a while as a treat.

But remove any uneaten food quickly as they are sensitive to dirty water.

Only HOB or canister filters should be used with crayfish, they tend to jam undergravel filters with all their burrowing.