Having a subtropical aquarium can be a unique experience for the hobbyist wanting to try something a little different. Here are some basic guidelines, benefits, and possible fish species you can try for one of these setups.
A subtropical aquarium mainly differs from a typical aquarium setup in that it doesn't necessarily require the use of a heater. These tanks are sometimes referred to as coldwater aquariums or, in some instances, river aquariums. True coldwater and river aquariums, however, often need to be colder than subtropical tanks. For this reason, it is important to keep track of the water temperature in your tank before adding fish, and researching the proper temperature range and other parameters for the fish you choose. (Fortunately, many species can exist in either). It's important to remember that your tanks should always be cycled before adding fish.
Many subtropical fish don't just tolerate lower temperatures, but prefer and thrive in them. Often, aquarium water kept at room temperature of a centrally-heated home will easily do the job.
Now, let's take a look at some subtropical fish and invertebrate species. Of course, preferences are different for everyone, and these are only suggestions of where to begin. They are by far not the only options available. We are going to discuss only some of the species here. (I included several of the more common species, then added a few not generally seen in other blogs or articles, as I wanted to provide a little more variety than what I've read).
For all the invertebrates listed below, remember to avoid any foods, medications, or plant fertilizers that contain copper, as copper is toxic to invertebrates. Many of these items includes copper or copper sulfate. Be sure to check the ingredients of anything you are putting in your aquarium.
For shrimp and crayfish, calcium in the aquarium will help with molting. Some fishkeepers will add cuttlebone in their tanks for this purpose.
Any article or blog you read on subtropical tanks will mention these. Usually first. They are extremely hardy fish and peaceful with each other and their tank mates. It is recommended to have a group of 12 or more of these, though you could begin with 6 if you wanted to try them out before adding more. Their ideal temperature is 60-72F.
These little guys will not harm your aquatic plants like some of the larger crayfish will. They're also peaceful and very hardy, as long as the water parameters remain stable. They prefer places to hide, especially during moulting. Dwarf crayfish will benefit from a little calcium in the water, which they will need during the moulting process. They can tolerate temperatures between 60 and 82F, though I have always had more success keeping them in the 60s to low 70s than in the higher end. Your mileage may vary :)
The scientific name for cherry shrimp is Neocaridina davidi. The term cherry shrimp typically refers to the red variety of this dwarf shrimp species, though there are other colors available as well, such as yellow and orange. The dwarf shrimps have become very popular in the aquarium hobby, resulting in more and more types of shrimp being available. Cherry shrimps are hardy, adaptable, and will usually breed easily in the home aquarium. Many fish will eat the fry, however, if given the chance. Shrimp love a planted tank, as they primarily feed on biofilm, algae, and decaying plant matter; fry will also utilize plants for hiding. An ideal temperature range for shrimp is anywhere between 65 and 82F.
Red shiners are excellent for indoor unheated aquariums, as their temperature range is between 59 and 77F. They can reach up to 3.5"" in length, though usually stay a bit smaller. Red shiners are sometimes called rainbow daces. They are generally peaceful and can be housed with a number of active fish, but are fin nippers, so shouldn't be housed with goldfish and the like. As an active fish, these should be kept in groups - the larger the group, the happier they will be. Their color and activity make them an enjoyable choice.
The rosy bitterling is an uncommon subtropical fish, though they reach a maximum length of around 2.5 inches and require water temperatures to remain between 65 and 75F. This is a schooling, peaceful fish that needs good filtration and moderate to high water flow. A group of 6 or more is recommended.
The Pearl Galaxy Medaka Ricefis is sometimes also called Japanese Ricefish or Japanese Killifish, but any of the ricefish listed above would be good options for a subtropical aquarium. This is a schooling species, so a group of 6 or more is ideal. They require clean, well-maintained water, as well as a secure lid, as they are excellent jumpers. There is at least one other type of ricefish that have higher temperature needs, so it is best to make sure which rice fish you are housing. Their temperature range is 64-75F.
While not as colorful as the cherry shrimp and many of the shrimp cousins, the Amano shrimp is larger, active, and is known to eat algae. In fact, they are often called algae eating shrimp. Because of their complex breeding environment, they aren't usually, if ever, bred in the home aquarium. Their temperature range is between 60 and 80F.
There are may types of crayfish available for those interested in keeping this invertebrate. In general, however, the care guidelines are similar for all of them. Crayfish will typically eat whatever they can catch, so this should be considered if other tank mates are going to be added to the tank. Many crayfish will fight among each other. If more than one crayfish is in the same tank, it is recommended to have a large tank with plenty of places to hide. A solo crayfish will also require plenty of hiding places, especially for the moulting process. It's a good idea to provide additional calcium to aid in regrowth after shedding the exoskeleton.
Crayfish are also known escape artists, so a tightly secured lid is a must. Some are more reclusive, some are more active; it will depend on the specific crayfish. They make excellent scavengers; you should avoid housing them with other bottom dwelling fish. A good temperature range is between 65 and 75F.
These fish, also called anchor fish, stay small and have two areas of concern for those who keep them. First, they are very timid. It is often better to house these alone, as they will easily be outcompeted for food by many other fish species. Also, they require some extra oxygen in the tank. I have had much success with mini stone catfish by following these two guidelines. (I put four of them in a 5 gallon Fluval Chi, which offers water movement through the waterfall feature, and I added an air bubble wand. Numerous plants provided hiding places. Some days they move more than others, but it is an easy tank to maintain and fun to watch).
If you decide to keep these fish, keep in mind they are nocturnal, so may not move around much during the day. When they are active, they are a joy to watch. One benefit to keeping Asian Stone Mini Catfish is they don't require a lot of space, so they can be a good choice for the right nano tank. The ideal temperature is 64-75F.
Also called Paradise Gouramis, these are pretty and colorful fish and the males are known to fight among each other. It is best to keep only one male or a male and two to three females with plenty of space and places to hide. They can usually be housed with other types of fish. Keep in mind, though, that they have long fins and don't move quickly, so if kept with other fish, you need to be careful to avoid putting them with fin nippers. The Paradise Gourami needs to be in a temperature range between 65 and 75F.
Also called the Butterfly Loach, these have a different shape than most other fishes and don't generally get larger than about 2 inches. However, they need high amounts of oxygen and a well-established tank to thrive. They naturally come from fast-moving streams that are rich in oxygen. Therefore, it's highly recommended to use powerheads in their aquariums and a 10-15 times gallon per hour turnover. Because of their high need for oxygen, they are unlikely to survive in temperatures above 75F.
They do best in groups of 3 or more, and need very clean water. Hillstream loaches are omnivores that, along with regular fish food, will also feed on algae and biofilm, which is why a well-established tank is recommended. You may also want to consider a reticulated hillstream loach, which has a different pattern on the same body shape. The temperature should remain between 68 and 75F.
If you're still looking for something a little more unique, you may consider an axolotl, which is a fully aquatic salamander that has become extremely populer in the pet trade over the past few years. If this is more up your alley, feel free to read our blog about their care here.
Tropical Fish Hobbyist Magazine has an article here if you want to read more about subtropical aquariums in general, as well as some other fish possibilities.
This page provides information about coldwater aquariums.