Just lately the water of my terrapin tank has been a bit cloudy a few days after changing the water. Do I need to change or add anything or is my pump not big enough? Any suggestions?

Depends what you mean by cloudy.. my RES's tank water is usually not spring water clear after a day or so

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poor visabilty like light fog

Am using york stone as a platform and have about 130l water area and using a fluval 3+ pump

If your filter is an internal one the problem is almost certainly insufficient filtration, internal filters don't have the neccessary capacity to deal with turtle waste and do litle more than move the water around.
A large external filter, rated for 2-3 times the water capacity, will keep the water crystal clear with only minimal partial water changes.

What size and species is your turt BTW?

about 3inches and yellow belly.

How do you tell what sex there are?

What Filter make would you recomend?

If it's a Yellow Belly (YBS) then you can tell the sex by looking at the tail and the front claws, females have a shorter, thinner, and more pointed tail, with the cloaca roughly in line with the back edge of the carapace. Males have thicker and longer tails, less pointed, and the cloaca is nearer the tip. Sexually mature males also develop much longer front claws than females, sometimes up to an inch long.

Of course if you don't have a male and a female to be able to compare them it may not be so obvious, so you'll need to check against photos of other YBS. But at only 3" it's unlikely that you'll be able to sex it with any certainty, 3 or 4 years is about the earliest you can tell for sure.

If you have 130 litres of water that's pretty good for a turt this size, but you should be looking to increase that quite soon to keep in line with the recommended 40 litres per inch minimum. A greater volume of water not only gives them more swimming space, it's also a lot easier to keep clean as waste products are more diluted.

I can thoroughly recommend the Eheim 2217 filter which is adequate for turt tanks up to 300 litres, especially if you only have one turt in the tank as you do. You can currently buy this filter from Pet Supplies, Pet Food, and Pet Products on Sale Now at zooplus.co.uk for just under £60 (if it's your first order from them) including courier delivery from Germany.

I bought one recently and it's a great filter, very quiet, easy to set up, keeps the water very clean, and it comes with everything you need except a few kilos of bio-media which is cheap enough, I recommend Alfagrog from any good Koi supplier.

good filtration is very important internal filter like already mentioned are no good you need a external i have a under tray as well
good luck