Hello; I have a 55 gallon with several tiger barbs, zebra dainoes, serapae tetras and cherry barbs. These are fairly small fish but are often active, especially when I am around the tank and at feeding time.
The zebra danioes are the most active, very hardy and are tolerant of a wide range of water and temperature conditions. A 75 gallon could hold a decent sized school of these fish. I had a school of around two hundred at one time I was growing out in a 125 many years ago. It was quite a sight to watch. The zebras can be kept with many other fish.
The tiger barbs and searape tetras are aggressive for their size and this will limit what they can be kept with a bit. The combination I have described has worked well for several years. I started with ten small tiger barbs about four or five years ago and they are now mature and are the dominant group in the tank.
I have had some kuhli loaches in the tank along with the others for a few years and they have gotten along well with the others. Three black and two banded kuhlis.
I tried two ottocinclus catfish a few weeks ago in the tank. I was not sure if they would fit in, so kept a close watch on things. They have so far not been attacked by any of the other fish and are doing well. I will give it some more time to be sure and will likely add a few more as the tank is planted heavly and there is much more algae than the two can deal with so far.
If you decide to have the larger fish, you will likely only be able to keep very few fish. In some cases only one and often no rooted plants. I went thru a phase of keeping the larger fish for a decade or so. I have been keeping the smaller species and planted tanks for the last couple. With a 75 you are going to be able to have most any of the smaller species you may want, but will be near the lower end of capacity for many of the larger species. Some large species cannot be kept well in a 75.
Also keep in mind that the cute reasonable sized fish found in pet stores can grow into monsters. Which ever way you decide to go, do some checking before getting a fish.
