What size tank is it and what type algae do you have? Some algae eaters stay relatively small (otos) and others get very large (common plecos). Also, different fish eat different types of algae so if you're getting it to clean up your tank, you first have to identify the algae.
Having said that, if you do have an algae problem, it's best to identify what's causing it and get rid of the algae in that fashion rather than buying a fish or invert to take care of it.
The best algae eaters I have ever had are bristlenoses, specifically Ancistrus cirrhosus. I have heard that some other species of Ancistrus are not as bothered with algae, but these guys certainly are superb, and active throughout the day too !
Grow to about 4-5" max, so don't know if that's suitable for your tank size.
I would think the BN would get along fine with the clown. If you have a clown though, why do you need another AE? Is it not eating the algae you have? If not, the BN probably won't either. If it's dreaded blue-green algae, nothing will eat it.
The most common cause of algae is over feeding and too much light, especially direct sunlight. Do you know what type algae you have? That is a real key to controlling it.
I had my tank cycling for a few months and it catches direct sunlight, so it had brown algae like you would not believe... I wasn't concerned though because I knew the solution... (drum roll please!) I got 3 very young (about 2cm long) BN and within a week my tank was clean, the glass was spotless and they had all gone obese without me feeding them once. I'd choose these little dudes over introducing manufactured products anyday.
Also a majority of the algae species indicate your doing alot of things right, not wrong, as the algae just uses excess nutrients from fish waste that the plants don't use up. Getting the right balance between plants and fish is essential in algae control. I don't advise throwing a bunch of things into the aquarium willy nilly, I think the most important thing I've learnt from aquaria is to experiment, wait and observe, as alot of things require quite a long time to come into effect. Ask 100 aquarists and you'll be bound to get 100 different answers though.
I got rid of my algae in my little tank (No algae eaters) through surface agitation and DIY Co2 faster than I got rid of it in my larger tank (Bristlenoses) the Bristlenoses don't really seem bothered about any algae other than the green stuff that grows on the glass, hair algae they refuse to touch. You might want to buy a small powerhead because I know that the Juwel filters don't cause much current (One of their selling points, however in this case it is not useful)
Hope this helps..