Well the bigger the better. I would have to say 7 gallons isn't very large but it will contribute to some water filtration. I have a 90 gallon and am considering a 55 gallon fuge.
You can setup fuges next to your tank or even above your tank. Many folks have set up 20 or 29 gallon fuges next to there larger tanks. Ideally you would like to have about 5x to 7x the volume turnover rate in your fuge.
In order to get a larger one in, I would have to remove a brace while putting it in. With 700 Lbs. of water above it, I'm a little worried. Can't put it next to it.
What's the smallest ratio people have working?
Over all it may depend on what style of fuge you are creating.
If with a deep sandbed for nitrate reduction then you will need to go larger.
If for pod production then you may be fine with 7 gallons.
If for macro algae then you will need to light it with a photo period, and for nutrient uptake you could use more room but you may see some benifits over a few months time.
Then there are many combinations of fuges depending on size and desired function.
I would research refugiums posted by Bang Guy's or Kip4130's name in the search function. Perhaps one of them will stop by and give there experience.
Kip had or has a small fuge under a 54 gallon tank that I remember but I do not remember what size it was. A 10 gallon or so I think.
bigger is better and just about anything is better than nothing but 7G for a 90 is really close to worthless and hardly worth the effort IMO.
I'd look into a temp brace to use while installing a larger one or a 2 piece arrangement.
Here's the problem. My stand only has 12 inches front to back between the supports. Tanks that are 12 inches have a support at the bottom that make them about 12 1/2 inches. It doesn't fit. That leaves me to go to 10 inches. The door opening is 20 1/2 inches, so a 15 high is the only tank I can get in there without mods to the stand. Changes to the stand are hard with 90 gallons of water setting on it.
Here's the question. If I don't go with the normal baffles on either end, I could use eggcreate and increase my water volume to about 10 gallons. That's a good increase, but I don't know if it's enough space.
If I could mod the stand a little I could get a 29 gallon in there. I'm limited to 18 inches of hight also, unless I get a different skimmer. If I got a 29 gallon tank in there I could get to about 20 gallons of water volume and might even be able to put the baffles in it. Do the baffles really do any good?
I put an xenia frag in my tank before. It died. I would like to be able to grow it, but not possitive why it died. Algae is easy to grow. Was hopeing to do some mangrove plants and calerpua also. Wanted to get a good mix of algaes.
If I can get the support out for a short time to get the tank in, I might be able to do a little dremel tool work and get a 29 in there. That's a lot of work. If it will mean the different in success and failure I will certainly so it.
If people could list there refuge water volume and tank size for me I would really appreciate it.
I don't even use a single baffle in my 29G fuge. It really depends on how much flow you're looking to put through it. I'm running 600 GPH trhough it.
I had a bit of a problem with the 15 running on the 29 so I filled a 3" acrylic tube with LR and let the overflow dump into it. Makes a good place to toss bags of media. Drilled holes around it near the bottom and put it in corner. The return pump sat in a basket about half way up the side of the tank. I ran about 100G through that. If you try too much, you'll prob need baffles to control the bubbles.
Could you place 2 verticle supports 16" off center each direction and remove the center to make room for a 29? I don't know how your stand is contructed so I'm just brainstorming.
Here is a picture from before I got it. Best picture of the stand I have.
I like the two tank idea. If I could get the right rubbermaid box, that might work and be fairly cheap.
Does anyone make a 15 gallon high acrylic tank? I would think that would be kind of expensive. I almost think I could get a 30x11x18 custom made for less.