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for starters i was planning on getting another tank the only wide open tank would be a 75 gallon now i have a fluval designed for 300 gal will that be good enough since the tank wont actually be completely full and this these fish aquariums have those overheard lights any idea where i could get a screen for the top so the lamps can sit

I'm not sure I fully understand. What tank do you have now and how many turtles? What model filter do you have now? You will want a screen but you might not want it to rest on top of it.

i have a 20 gal 2 turtles and a fluval designed for 300 gal

Do you know the turtle genders yet? If not, how big are they?

2 females 1 is full grown the other 4-5 inches

You might try a 100 gallon Rubbermaid stock tank. Your filter is strong enough for it, and they're cheaper than 75 gallon aquariums.

im not familiar with stock tanks what is that exactly

Large plastic or metal tanks used for livestock. They are very durable and are usually cheaper than an aquarium.

i live in an apt so that wouldnt be an option for me

Actually, it's sometimes a better option in an apartment. A 75 gallon aquarium is OK, but might be a little cramped as they get bigger. Just a heads up, aggression sometimes pops up when you move turtles to a new tank.

i also have a room mate so i dont think he would appreciate that and thanks for the heads up

Take a look at the build thread linked in my signature. I have a 75G tank too. I used some aluminum light diffuser (aka eggcrate) for a cover the light could pass through or sit on. The filter media thread on the equipment section of this forum has many sizing and media questions answered. With two turtles, you'll probably want to be in the 5x water turnover per hour. So if you have 35 gallons in your 75 gallon tank, 35 x 5 = 175 gallons per hour. Now that's actual water turnover and not just the pump rating meaning after all filter media which slows down water flow and after the head (height of the tubes or gravity the pump has to fight to push the water) you probably want something in the 175GpH range. You could aim for some sort of tank divider if aggression pops up until you figure out a better option.

thanks

so i ended up buying the 75 gal i have had it 3 weeks and it has still not cycled water is green now i know more water means it takes longer to cycle but how long should it take

Green is usually algae.