Using a hard rule of x horses to y gallons is not the best way to approach stocking a tank.
Stocking density really depends on two things: the nutrient processing capacity of your tank: the space requirements of your fish/horses.
A tank with skimmer, deep sand bed, refugium full of algae and reasonably sized weekly water changes will support infinitely more horses than a tank with no sand bed, skimmer, refugium...
In the wild, horses have territories between 300 and 1,000 sq feet. Thats somewhere between the size of a large bedroom and an apartment or small house. Lots of room for such a small critter. Now, they do not defend their territories and territories are known to overlap, but still...
If you are going to put six in a 30g, I would start with one pair and make sure the tank's capacity to process nutrients is ramped up before adding more.
To give you an example of what will happen to your water when you add your horses, I added 3 to a 40g tank (no skimmer) with live rock, a sand bed, and a small amount of macro algae, that had been set up for 4 months to adequately cycle.
Within several weeks my nitrates had gone from undetectable to 20 mg/l. Nitrites never showed as measurable, so I had a good bacterial population established to process them. Nitrates at that level are not a concern in the short term, but I would not want them that high in the long run.
After adding a rather large refugium (40g :eek1: )and as the macro algae started to grow my nitrates slowly reduced to undetectable.
So, consider the needs of your horses, the biological processing capacity of your tank, and go slow.
Fred
