When it comes to feeding, I am not scientific. You are supposed to feed an adult turtle every other or every 3rd day and by the time they are adult, they need a diet that is not only comprised of those little pellets you can buy at the pet store, but also leafy veggies. Turtles are notoriously messy eaters and as much as half the food they chew gets in their stomach and the other half floats around the tank. After years of practice, I don't measure how much I put in the tank, but kind of guesstimate what Sandy will eat. I know I've nailed it when I feed her and return 15 minutes later and find only a few scraps left over. No waste! Then I'll take a fish tank net and get those scraps out before they dissolve into the water. To make it easy, I usually feed her right when I wake up every 2 or 3 days, then take a shower, and return to scoop up the leftovers. If she has pooped, I'll scoop that up too before it dissolves. This goes a long way to keeping the water cleaner longer. When the turtle is young, it is suggested that you have one tank for the turtle to live in and another to feed it. When turtles are small, you can get away with 2 tanks, but when they are bigger, the amount of area and the risk of the thing biting or scratching you to pick it up makes it unfeasible.
Filtering the water is very important. You have to remember that the water in a turtle tank serves as a drinking supply and toilet at the same time. Carbon filters capture and neutralize most of the excrement to keep the water at a 'potable' level. The first lesson you learn when you own fish, turtles, or other aquatic animals is that the expectation should be that the filter keeps the water safe for the animal for a period of time, not that it keeps the water perfectly clear indefinitely. Turtle poo is especially putrid and viscous and will clog your beautiful filtration system. No matter what, the water will become visibly brown and cloudy after a time. What do you do then? Well, change out the water, of course! For me and my set up, I completely empty the tank and refill it with fresh water once a month. That means that I scoop up the water a little at a time, pour it down the toilet, heat up fresh water, and refill the tank. This process takes me up to 4 hours, start to finish. Also, while I do this 'deep clean', I take out the basking platform, filters, heater, and anything else in the tank and scrub all the excrement off of them. In the case of the filters, I completely take them apart and get all the goo out of the motor as well. This is the cost of having a turtle as a pet: Time.
The filters themselves are very important. I use two, very basic filters that I found at Walmart. They are cheap, expendable, and easy to open and clean. In previous setups, I bought fancy canister filters and others, but they are expensive, hard to get started sometimes, and get clogged easily. Simple is good. Using PVC extensions on either side of my basking platform, I have been able to bring the filters close to the water level. I also bought versions that are rated to move more water per hour than exists in the tank. This is important because, as the filters age and dirty up, they lose effectiveness. At the worst, the filters need to move as much water as there is in the tank at least once an hour. Eventually, the filters break, but that's alright: Just throw it out and buy a new one.
Creating a basking platform is a common problem that turtle owners have to tackle. Turtles absolutely need to get out of the water for stretches of time to absorb vitamins and metabolize their food. I've had a number of different setups. When the turtle is young, they are small enough to climb up on one of these floating docks, which are very convenient as water evaporates out the tank. After Sandy grew too heavy for the floating dock, I bought this contraption called a Turtle Topper which worked fantastically. Unfortunately, Sandy outgrew that as well and there was nothing bigger on the market. That's when I turned to DIY methods. A simple Google search will yield hundreds of homemade basking solutions. I made mine out of a PVC frame, the ramp from the old Turtle Topper, and a plastic cutting board. As mentioned previously, I also incorporated extensions on either side of the platform to hold up the filters. It works very well, but soon I will have to go back to the drawing board to expand the tank for the ever-growing reptile!
For lighting, I have used many fixtures and bulbs over the many habitats I have created. It is suggested that the basking platform include a heat lamp, but in lieu of that, I placed my tank next to a very sunny window and that, together with the full-spectrum fluorescent, is enough for Sandy to stay warm. Keep in mind that glass and windows kill much of the good UVA and UVB out of light (or so I've been told). Similarly, full-spectrum light can only penetrate so far into water. I believe the limit may be a foot of water depending on its clarity and other factors. It's also very important to remember that bulbs have an effective life span of about 6 or 7 months. That means that even though the bulb still turns on and doesn't appear any different to your eye, you still should buy a new full-spectrum bulb after about 6 months. It sucks because they are a little pricey, but it's for the good of the turtle and as generally uncaring and cold-blooded as reptiles are, they will thank you.