Tropical and 'Cold Water' Fishes Together?

It has been always told to keep tropical with tropical and to keep cold water with cold water. This is true and correct. However, there are some ways to go around this rule. One can keep tropical and cold water fish together in such a peaceful way. With this type of approach, the hobbyist have better variety on the contents of his fish tanks. Cold water fish like goldfish have been placed in planted tanks with no ill effect. Then, why not go a step further and put the goldfish in a tropical planted tank?

Assuming the hobbyist is already aware of the considerations in size, food and behavior; what else should he consider in order to keep these two different classifications of fish?

It is best to have fish that are both born and raised in the same country. Why? It is because both fish have adapted to the same climate and same waters which make it easier for them to go together. Similar parameters mean they can share the same aquarium.

If the optimal consideration above is not met, the fish hobbyist can opt to slowly acclimatize both to a common parameter then slowly adjust to a warmer temperature. It is better to keep fish in warmer temperature for them to be less likely prone to infections. Also, the cold water fish will be more active than in the usual cold water environment they used to be. However, since the fish are more active, the filter needs to level up and clean the additional waste created.

Here is a rough guide on slowly acclimatizing the fish:
1. Start of with using water from the same source (not only the same tap but also the same aging tub) during water changes.
2. Place the aquariums beside each other. This is unnecessary if the tanks share the same room.
3. Exchange the two tanks' waters. 2gal (less if the one or both tanks are less than 5gallons) every 15minutes and do this thrice every 2 days.
4. After 2-3 weeks, you can try joining the fish together. Add 1 fish first and observe. If the fish is doing well, then transfer the rest.

Raising Mixed Spawns

Having many fry and few tanks is a problem especially if the fry are not yet at the point where one can choose the best and cull the rest nor is it at the point where they can be sold or given away. Or maybe one just wants to make space for other fish and fry. One may resort to raising mixed spawns. It can be one of four cases:
Case 1. Same species, same strain just the different birth dates and/or parents
Case 2. Same species, different strains
Case 3. Different species
Case 4. Combinations of the other three cases.


Here are some tips and considerations:
Case 1. There is little to no chance of differentiating. Size is a clue but not always because there can be runts and shooters in the spawns thus obscuring any size differentiation. However, for genetics sake, keeping fry of the same parents together is recommended.