External, good to about 75-80 gallon, more if you add a larger pump run 45$ includes shipping, pump etc (for a new one which I don't have ay the moment). I only have 2 left, it'll be after xmas before I have more. But I'll have another 10 after X mas with pumps etc. I have a couple of used ones (35$)
and a sump model which is good to about 300 gallons or so (without pump) for 45$. The sump types have never needed to be cleaned and have minimal pressure to run them and both types can be used with DIY yeast set ups.
I have a few internals left(4, 2 new, two used) which are 20$/10$ ea(20$ used w/used pump).
Plug the pump into the light timer.
At night when the pump stops, the mulm plant leaves etc fall off or float up and it doesn't ever clog. Also, allows CO2 to be added only when you need it. You don't need it at night and gain NOTHING from adding it 24/7.
Doubles the pump's life and saves electric and CO2 if you use it in conjuction with a solenoid. The increase in pH at night due to the water 's KH has little effect on the fauna and flora. The idea that it somehow is better and stanbilizes a tank better is not true in practical terms in any and every tank I've ever seen or had or heard of.
I clean it out once every 6 months or so. Hot water and a little bleach run through it.
You can try the night time off method to see if that prevents clogging.
Regards,
Tom Barr