Hi,
I bought a HOB wet/dry system. It's a used one and comes w/ bioball, overflow and pump. But I want to replace the pump. My lfs suggests lower gph for Wet / Dry system. When I check online-store, they are selling wet/dry with at least 500 gph. What is the appropriate gph for wet/dry system? Thank you.

I think it partly depends on what your overflow can handle. My SOS overflow is rated for 600 gph. Anything over the limit on the overflow could get very messy.

Mine is 600 gph also. Should I go for 500 gph? LFS said Bacteria needs time to consume.

Im sorry, I am completely clueless about these things. Fred, is that what you had hanging on the side of your 90 gallon? What are the benefits of having these over regular(HOB) filters?

Yeah, this is the one from the side of 90 gallons.

Wet/Dry is a bio filter, somebody correct me if I am wrong. In my situation, I change 90% every week. Sometime HOB is getting dry during the process. I am afraid I will break my cycle in the future. Wet/Dry always has water in the chamber to maintain the cycle (becoz of overflow). I can use cannister as well.
On the other hand, I got it from Craigslist w/ a very cheap price. $80 (after delivered) I think regular price is about $200.

Sounds like something I should look into in the filter. Either that or a canister. So far I have only used HOB filters. Something additional would be nice. Thanks for the explanation Fred.

I am using a Pondmaster pump rated at 950 gph. The exercise I went through was something like this:

1) I wanted to maximize flow through sump. Since the limiting factor is the overflow box the max amount of flow is 600 gph.
2) Calculate the head feet of the installation. This can be complicated or you can do a best guess type of thingy. Add one foot for each foot between the pump and the outlet in the tank. Add another foot for every 10 feet of tubing. Add another foot for every 90 degree turn in the line. (See http://www.pond-supplies-and-help.com/head-height.html).
3) Look at the specifications for the pumps to determine which one will give you the flow you want at the head height of your installation.

Note that I tried to maximize the flow through my sump. Maybe that wasn't the right thing to do

Jim,
Thanks for the height calculation method. I am lucky that I don't need a powerful pump since my W/D is a HOB type. It is only 1 or 2 feet from the sump to the max level.

Is it true, Beneficial Bacteria need a slow flow rate to consume ammonia, nitrite?

Hmmm... I'm not an expert but I'll give you my two cents worth

It seems to me that a slow flow rate would equate to more air being involved in the nitrification process and that this would be a good thing. I believe this is one of the characteristics of wet/dry systems that make it such a good biological filter. But there are other ways to increase the amount of air involved. The amount of media for one. The more media you have the more water you can pump through the system and still maintain a high 'dry' aspect to the process. My sump has quite a bit of media and a panel with holes in it that distributes the water across the media fairly evenly. So, even at something close to 600 gph I am still maintaining a high level of air in the sump. (I also have a dual air pump providing air to the sump). I have heard others also speak of 'dwell' time. i.e. the more dwell time the water has on the media the better. It doesn't seem to me that the flow rate would have any impact on that though. It seems more likely that the depth of the media is the only way to increase the dwell time.

Sorry that I can't provide more expert opinion.

Thanks Jim.