Aquarium Info

I have a 20 gallon freshwater tank for over a year now. The problem is, every time i add new fish they die. Usually, the fish hangs out in the corners of the tank and at the bottom, they don’t swim around like they should. I have 1 male swordtail and 1 loach. They are the only fish that have survived for a descent amount of time. I believe that the amonia, nitrate, etc. levels are fine; however, I dont have a kit but I only have 2 fish and I keep up on partial water changes. The pH level is alkaline but it should be in range for the swordtails I do add and from what I understand, they are hardy fish. In addition, the plants that I have in my tank are surviving but their not really taking off like they should be. Is it possible that the pH is causing the problem? If it is how can I fix this? And other than pH, what else could be causing new fish to die in my tank and what can I do to keep these fish alive in the future? Any help would be much appreciated.
I do acclimate the fish before I put them in and I have an underground filter, sorry, I probably should have said that before. It is usually afer a few days that they die on me. Occasionally some of the new fish get ich but currently none of the new ones i added today or the old ones have it. I rescently treated the tank for it though.

Enjoy guys.

KAHIN DOOOR JAB DIN DHHAL JAYE, SHHAM KI DULHAN BADAN CHURAYE,CHUPKE SE AAYE ,MERE KHAYALON KE AANGAN MEIN ,KOI SAPNON KE ,DEEP JALAYE DEEP JALAYE..
DIL JAANE MERE SARE BHED YEH GEHRE ,HO GAYE KAISE MERE SAPNEY SUNHEREY,YEH MERE SAPNEY YAHI TOH HAI APNE ,INSE JUDA NA HONGE GAM KE YEH SAAYE GAM KE YE SAAYE,KAHIN DOOOR JAB DIN DHHAL JAYE

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