Was in the same boat , no spring chicken here ! I always made my own filters when younger but now being flat out old ,well. I done a lot of homework ! For turtles its the flow rate and volume of media also type of media. There is truly only one filter for us , Fluval FX series. The four series could work for you but the FX6 is the last filter you will need. With the holidays coming around should be on sale soon. You will find the 6 is on sale more often than the 4 and sometimes cheaper than the 4. The 6 has a filtration volume of 5.28 gallons , that's massive. With a flow rate of 563gph that's rated with the media. Yes large in size but comes with a built in drain so you can empty it to be lighter. Line is short so at the hardware I got a longer one to make it out the front door. If I needed to I could go every four weeks between cleanings but I do every 3. If push came to shove could go longer , you know on those weeks we are having a hard time. The only thing you have to think about is the carbon you use , depending on the load on it some cheap carbon will only last 4-5 weeks then gets saturated needing changing. I'm old and handicap and have no problem with it. If I feel the need I could put it on a dolly or install swivel wheels on it easy. I use the FX6 on 125 gallons for water and it's great ! Some say the nozzle has to much flow . You as I can play with it play with it's placement , remove the diffuser completely to go with open pipe. Or just crank filter down a little with the supplied valve. As you know if cranking down a filter do it on the outlet side never the inlet side. This was the last video I made a few weeks ago , will give an idea of how well it cleans.
https://youtu.be/Ca7w2FCMSyIIt's easy to clean all in one place , not like all the separate DIY's I had , you know like a 30 gallon bio tank a mechanical tank and then the chemical section was lots of work. This has the largest media section I could find that actually works very well with a great flow. My media is setup : the foam pads , bio rings , carbon and purgein .
http://www.fluvalaquatics.com/us/produc ... eiGXbpFxok