Hey there. I know you pm'd me, but since you also started this thread I'm going to respond here so other new members can read and learn too.
First, let's talk tank size. In my estimation, nothing less than a 29g tank should be considered for H. erectus and H. reidi horses....ESPECIALLY for new horse owners. More advanced hobbyists who are more familiar with the ins and outs of seahorse care may be able to keep horses long term in 20g tanks, but I think you'll find that these people have also learned enough to know that it's not a good idea to DO for long term. The smallest tank I would personally recommend is a 29g tank, with a tall tank in the thirties more preferable. But you can safely keep a pair of H. erectus in a 29 without any problems.
Now let's talk about plumbing. It is a very bad idea to share this tank's plumbing with the bigger tank. First off, you will be sharing potential pathogens from one tank to the other, and these may be diseases the horses haven't been exposed to and can't handle. Just as important...you REALLY should not keep a seahorse tank at 79-80. 75 is actually near the top of the range for these guys, and you'll be much better off keeping it lower than that.
Now, here's some good news. You don't want to spend an arm and a leg on a chiller. Don't blame ya. But I would suggest you shell out $100 for a piece of equipment called an Ice Probe, and another $15 for an Aquaclear filter to fit it to. This little mini chiller will (as long as you're using T5s or PC lighting!!) easily keep a 29g tank in the 73 degree range. I used one on my 37g tank while I used PC lights to great effect.
Now, let's talk about your tank plan. I think your idea of using Macros is a good move on your part. Macros are a helpful and easy way of keeping the nitrates down in the tank. You mentioned in your PM, "How clean does the water need to be," and talked about corals that can handle dirty water. I don't want you to think that Horses should be in dirty water conditions. It IS true that seahorse tanks are typically higher in nitrates than your average reef tank, but this isn't something that should be accepted....it's a part of our chemistry balance that we're always fighting. We combat it with the use of regular water changes, macroalgae, and whatever various nitrate-reducing WonderProducts we happen to be in love with that particular month.
My personal regimen is 5 gallons WC a week, and full filter pad change every 2 weeks. I think that many Caulerpa macros like C. racemosa and C. mexicana are great for nutrient export in horse tanks as long as they are harvested properly and on a regular basis.
Since you're looking to keep your bills down, I think you'll be pleased to know that you don't HAVE to keep any super-expensive filtration on this. Sump style filters or canisters may be nice, but not necessary. A simple HOB style filter (rated for, say, 55g) will suffice along with the Aquaclear with the iceprobe fitting. A heater is hardly necessary down here in Florida, except for our cooler winter days, and a simple air pump with a rigid air line (no air stone) will help move water as well.
You didn't mention Protein Skimming. IMO this is one of the most necessary ingredients in a Horse tank, and one that shouldn't be skimped on. I used to have the Aquaeruo USA nano skimmer (before they changed the design) and it was a decent little skimmer for a very reasonable price. I've since upped to the Tunze nano, and I have to say I love it. Might be too much of an eyesore in a 29g tank, though...you will have to decide that.
Finally, fish. Yes, there are a good number of fish that are safe to house with horses. There's a whole list that's stickied on this site, so I'm not going to wast time repeating it. I will say this, though: Since you're new to horses, start with just the horses. Don't go fiddling around with new fish until you have a few months of seahorse maintenance under your belt. Once you have that knowledge and experience of how horse tanks operate, then you can be free to consider adding a small firefish or other SLOW eating animal....but get your feet wet with just the horses before you go there.
I hope that's a good start for you! There are plenty of reference threads on this site for seahorse beginners. I would efinitely take an evening and read through them carefully....and of course....have fun!
