Instead of lettuce, try feeding it collared greens, carrots, a bit of kale, etc (just feed veggies like chinese cabage & spinach VERY sparingly, because of the oxalic acids that interfere with calcium absorbsion). The reason 4 the soft shell is most likely MBD (Metabolic Bone Disease -> the animal is not getting enough calcium & vitamin D3). Get it to a herp vet ASAP. Also feed it some cuttlebone (usually used 4 birds) as a calcium supplement. Besides that, also feed em crickets, mealworms (not to much, as they have a bad phosphour : calcium rate), earthworms (excelent food item), not to forget to gutload em. Fruits are also fine (not citrus fruits however). And, also feed some feeder fish (small goldfish & guppies, just make sure that they're parasite free & well fed a few days before feeding), and some dried/feeder shrimp (if they are available). Be persistent when feeding pellets, maybe some thay he/she will find them irresistable. Oh yeah, and not to forget to provide your pet with ultraviolet lighting (sunlight is best, & free too
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My "babies" :
1.0.0 Indo BTS (Tiliqua gigas gigas)
0.0.1 Rainbow skinks (Mabuya multifasciata)
0.0.3 RES (Trachemys scripta elegans)
0.0.4 Amboina box turtles (Cuora amboinensis)
1.0.0 Chinese softshell (Trionyx sinensis)
1.0.0 Asian leaf turtle (Cyclemys dentata)
1.0.0 Leopard gecko (Eublepharis macularius)
And still growing...