Question about tank size

[FishInABarrel]

My wife has wanted seahorses since I got into saltwater but I told her they need a dedicated tank. A buddy of mine was moving and gave me what we thought was a 29g that I told her I'd set up for seahorses. After I drilled it and was looking at it, it didn't quite seem big enough to be 29g. After checking measurements it turns out to be a 25g.

So my question is, is 30g the minimum for the footprint or the water volume? Can I put a pair of seahorses in the 25g with a 10g sump?

 

[rayjay]

IMO, yes you can.
The sump will probably add the extra 4 to 5g to bring you up a bit.
However, to be on the safe side, just be extra diligent with water changes and husbandry over normal seahorse needs, which in turn are more than the needs of a reef tank.
If you could add a skimmer suitable for a 100g tank or larger it would help a lot.
You can find some basics on their needs at "My Thoughts on Seahorse Keeping"

 

[FishInABarrel]

Ok. If it's mainly the water volume and/or liverock amount, I do have a spare 20g as well. I do have a Aqua medic turboflotor multi sl I could use if I'm going to need as much skimming as possible.

 

[rayjay]

Water volume is indeed the big plus to help in success of seahorse keeping.
Seahorses, by nature foul the water quite bad considering they choose only the most appealing pieces of food (at least in their eyes) leaving the remaining pieces to get trapped usually out of sight under/around rocks and in and around decor/hitches and in filters. This needs to be removed on a very regular basis to prevent nasty bacteria cultures live the vibrios from getting out of hand as they use this detritus as bedding sites and as food to propagate.
In addition, when a seahorse snicks up it's food, it gets masticated and minute particulate matter is expelled through the gills and into the water (sometimes you will see a "cloud" emanating from the gills upon snicking).
This further degrades the water and provides food for the nasty bacteria.
Hobby test kits CANNOT tell you when the water is dirty enough to be a problem for seahorses so it's a case of proactive husbandry.
IMO, that skimmer will be a big aid for you in the seahorse tank.