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I have 3 red eared sliders. Biggest turtle is 20 years old and is roughly the size of a stretched hand. Other 2 are much smaller and are 3 years old or younger.
I am looking at 75 Gallon tank with dimensions of 48'' x 18'' x 20''(length x width x height)
Would this be suitable or should I get a bigger tank.

Any suggestions would be greatly appreciated.

How long you had the turtles or did you raise them from babies ? Adopted or rescue ? What is the sex of the 20 year old being about 6" ? If male may be full grown but if female can reach 12" needing a much larger tank "alone". Other two may not be sexual mature yet to tell sex to get a final home yet. All three in a 75 gallon to me will be a very large bio load on any filter. Resulting in lots more maintenance and more often water changes. You may know how we all just love doing maintenance. Haha ! That's a confined space for three even if filled full with an "ATBA". Upfront if 20yr old a male 6" the 75 gallon would be great for him alone . Then another tank for the two smaller ones being about the same size. They may get along for awhile until sexual maturity. Turtles are territorial , aggression will happen in small tanks and yes to me 75 is small for three turtles one being an adult. You may get lucky with the smaller ones about same size in another tank , then sex will matter down the road , if separation needed for them also. Normally 10 gallons of water for every inch of shell works. This is not just for more swimming room but helps a lot to control the waters parameters. You don't want small turtles in with the larger 6" , things will happen. You can not watch them 24/7 ! The personality's , aggression , sex all matter to have a happy setup. Yes some people do have three turtles in one "large" glass or stock tank , lots of factors must come together for it to work. Also keep in mind when all are adults in size , that's going to be large tanks and maybe two or three depending on the separations needed.

I've got the same question, regarding 2 relatively young (exact age unknown, less than 4 years old) males, both around 3-4 inches in length, what would be the ideal tank size? Right now they're housed in a 150 liter aquarium that I had lying around, but it feels quite small for them.

75 gallons would be nice, but they are still growing. One main thing to consider is that males often do not get along... you might have to separate them in the future.

What are the sexes of them ? That size they should be sexually mature to tell. Depending on that you may need two tanks for separation or one large that you can divide safely. It's recommended to have 10 gallons of water alone for every in of shell. This is not just for more swimming room but so you will be able to control the waters parameters which helps with the maintenance .

The 150liter / 40 gallon you have now may work just for one turtle that size today but will out grow it soon. You could if one is a small male could build an "ATBA" and fill it full to buy some more time. Then another tank for the second.

You may want to make post as a "new topic" so different poster's do not get confused on details that's for the other. Keep all clear.