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Marbled Newts (Triturus marmoratus)
Marbled newts are a large, colorful European species of newt in the Triturus genus. They are generally more robust in build than other crested newts and have a characteristic bright green and black mottled appearance. There is a similar species, Triturus pygmaeus that is smaller but with essentially the same care requirements.
Marbled newts typically reach a maximum size of about five or six inches in total length and are fairly active. Like many newts, the adults are primarily aquatic animals but appreciate a place to haul out of the water. For a single newt, a ten or twenty gallon aquarium would be sufficient (but bigger is always better), and a pair or trio could comfortably be housed in a twenty long or larger. They are generally not aggressive to each other but accidents may happen (newts can be aggressive to anything grabbed while feeding).
Recently morphed marbled newts will need a terrestrial set up, as they spend approximately a year as "efts" before returning to water to breed. You can provide them with a simple plastic critter keeper or similar with damp sphagnum moss as substrate. During this time they will eat only live foods such as pinhead crickets, white worms, chopped earthworms and fruit flies.
 Make sure to secure the lid of the aquarium/terrarium, preferably with a tightly fitting screen top as they can climb. You can create a land area for the newts by sloping gravel to one side of the tank, or by using a floating piece of cork bark. Filtration can be as simple as a sponge filter attached to an air pump, since they prefer water with very little flow. Temperature should ideally be around 65 F. They can tolerate a bit warmer in the summer but the cooler the better. Do partial water changes at least every two weeks to keep the water from building up harmful nitrogenous waste. When replacing with clean water, make sure to treat with a dechlorinator product.
Newts do not need any sort of special lighting, but if you choose to grow live plants in the enclosure, the plants may. Lighting is however, helpful in triggering newts to breed, so read on with regard to that.
Marbled newts are not picky eaters. Adults will eat commercially prepared sinking pellet fish foods, frozen blood worms and live worms of almost any kind. (However, be careful when acquiring wild worms from outside, as they may contain trace amounts of pesticides and other harmful things). Offering a variety of foods ensures good nutrition for your newts so mixing it up is recommended. Feed newts every two to three days and siphon out any uneaten food before it rots.
Marbled newts are an easy species to breed and during the breeding season, a very easy species to differentiate males from females. Males will develop a short crest and often a white lateral stripe running down the body. Females are generally larger, rounder in the belly and darker in coloration. Often these newts will begin to breed if simply exposed to natural sunlight and a drop in temperature. 
Once breeding starts, you can give them material to begin laying eggs on. Marbled newts in the wild use aquatic vegetation to fold each individual egg into. Unlike their cousins, the Danube crested newts (which will happily use strips of plastic), marbled newts tend to insist on live plants to lay their eggs into. Java fern and elodea make good laying material. Some hobbyists have also had luck using pieces of yarn or the plastic "grass" sold for easter egg baskets. Triturus newts have a naturally occurring genetic defect where roughly a third of any given clutch will fail to develop past the tail bud stage. This is totally normal for them. You can remove any eggs that appear cloudy or fuzzy as they are dead and are developing fungus that can spread to the other eggs. I remove the eggs from the parent tank, since Triturus newts tend to eat their own young.
Once the eggs hatch, the larvae will appear as tiny yellow and black striped tadpoles. The best thing to feed them is pond water from outside if you have access to such a thing. There's no comparison to the microorganisms that are in there. If you don't have access to a supply of pond water, live daphnia, live black worms that are chopped up with a razor blade will work, as will live baby brine shrimp. The larvae like to hide in surface vegetation, so a nursery tank with floating plants is ideal. These newts are fast growers compared to some other species, and will start to develop front limbs in a matter of weeks. After a month to two months, when the back legs come in, you can begin offering non-live foods just like the adults would eat.
Marbled newts generally take about two years to reach sexual maturity. They may try to breed at one year old, but often will be outcompeted by the older more impressive males.
A Word About Mixing Species
Many people ask if they can mix X species with Y species. My answer is almost always, no. These animals should be kept in a species specific setup for their own safety and the safety of anything else. They will generally try to eat anything they can fit in their mouth (and sometimes things they cannot), and the risk of potential diseases being transmissible between amphibians is well documented. That said, I have safely kept marbled newts with small snails, and live plants such as java moss, java fern, green myrio and elodea.